Silicone Pressure Sensitive Adhesives

May 18, 2004

Silicone Pressure Sensitive A

dhesives

Edward M. Petrie, Member of SpecialChem Technical E

xpert Team.

Introduction

Silicone Compared to Competitive PSAs

Applications

Raw Materials

Cure Mechanisms

Additives

New Forms of Silicone PSAs

Introduction

Silicone adhesives fill small but important niches in the

pre

ssure sensitive adhesive

(PSA)

market. They are generally selected for the most demanding industrial applications because of

their inherent resistance to temperature extremes, moisture, chemicals, and biological attack.

Silicone adhesives also offer excellent dielectric properties so that they are often used as

pressure sensitive

tapes for the electrical / electronics industry.

Silicone polymers have an inorganic backbone and side groups that are organic. These

adhesives are expensive relative to other types of PSAs, and this is the main

reason why they

are used only in niche markets. Although silicone PSAs provide premium performance, they

generally require special formulation, as do other PSAs, to be accurately matched to specific

application requirements.

This article will review the formulation and applications of silicone

pre

ssure sensitive

adhesives. The raw materials used in their manufacture will be examined as well as

formulation possibilities to meet specific properties. The various silicone PSA cure

mechanisms will also be described. Finally, new forms of silicone PSAs will be discussed

relative to their market drivers.

Silicone Compared to Competitive PSAs

Pressure sensitive

adhesives generally come in three chemistries depending on their base

polymers: rubber-based, acrylic, and silicone. Table 1 provides a general comparison of these

three

adhe

sive

systems.

Property

Cost

Rubber-Based

Low

est

Acrylic

Medium

Silicone

Very High

Tack

180°

Peel S

trength

Shear S

trength

Humidity R

esistance

Service T

emperature

Service E

nvironment

UV Resistance

Adhesive Color

Solvent / Chemical R

esistance

Plasticizer R

esistance

T

ypically high

Low-to-high

T

ypically low

Low-to-moderate

Moderate

E

xcellent

-73 to 250°

C

Moderate-to-high Low-to-high

Low-to-high

E

xcellent

0 to 65°

C

Indoor

P

oor

Yellow

P

oor

T

ypically poor

Low-to-high

E

xcellent

-40 to 150°

C

Indoor or outdoor Indoor or outdoor

E

xcellent

Clear-to-straw

Good

P

oor-to-fair

E

xcellent

P

oor-to-high

E

xcellent

Clear

E

xcellent

Good

E

xcellent

High

Bond to Higher E

nergy Substrates

E

xcellent

Bond to Low

E

nergy S

ubstrates

Moderate

Table 1:

Comparison of Conventional

Pressure S

ensitive

Adhesi

v

es

Rubber-based PSAs adhere to a wider variety of surfaces than do acrylic adhesives, although

they have a narrower service temperature range and do not resist UV as well. Rubber-based

adhesives are highly compounded, and they are typically the best and most cost effective

choice for less demanding applications.

The acrylic PSAs fill the gap between silicones and rubber-based adhesives in terms of cost

and performance. They require fewer additives than rubber-based

adhesive

for optimal

performance, and acrylic PSAs provide many advantages over rubber- based PSAs. They

exhibit outstanding tack and peel strength and are second only to silicone in terms of yellowing

and oxidations. Therefore, acrylic PSAs are one of the first choices for outdoor applications.

Silicone formulations have the highest temperature and chemical resistance by far, but they

are expensive and provide less aggressive bonds than do acrylics or rubber-based adhesives.

Silicone PSAs generally demonstrate lower tack and peel strength than other

adhesive

systems.

However, they provide outstanding temperature and chemical resistance due to their inorganic

nature. They have very low glass transition temperatures and, therefore, can be utilized at

service temperatures between -73 and 250°

C without becoming brittle or drying out. Because

of their relatively low surface energy, about 21 mJ/m

2

, silicone PSAs will bond well to both

high- and low-energy surfaces, including etched polytetrafluoroethylene and unetched

polyolefins, polyester, and fluorohalocarbon films. Silicone PSAs are the only

adhesive

that

will consistently bond to silicone substrates, so they are often used as a

pre

ssure sensitive

adhesive

on extruded silicone rubber gasketing.

Applications

Silicone PSAs are the preferred

adhesive

in many applications that must resist harsh

conditions (i.e., high temperatures and chemicals) during processing. Table 2 shows the effect

of temperature on the peel strength of a silicone PSA tape. Although not outstanding, peel

strength is maintained up to 250°

C. Silicone polymers are resistant to processes such as

electronic assembly, plasma / flame spraying, and painting. They are also resistant to

moisture, most chemicals, and biological attack.

Adhesive

Strength, oz/in

Testing T

emperature /

Condition

-70°

C

-20°

C

100?

150°

C

200?

250?

Heat

aging

prior

to

testing

at

25°

C

No aging

1 hr at 150°

C

24 hr at 150°

C

7 days at 150°

C

1 hr at 250°

C

24 hr at 250°

C

7 days at 250°

C

Uncatalyzed

adhesive

>100

>100

60

60

Cohesive failure

Cohesive failure

60

55

60

70

60

65

Cohesive failure

Table 2:

E

ffect of T

emperature and

A

ging on Silicone

Pressure S

ensitive

T

ape (aluminum foil

backing)

1

Catalyzed

adhesive

(1% benzoyl

peroxide)

>100

>100

48

45

40

35

52

50

50

50

50

65

65

Perhaps the highest volume use of silicone

pressure sensitive

adhesives is in tape

construction. Several of the more common PSA tape applications are summarized in Table 3.

Compared to other PSA adhesives, silicone tapes can more easily withstand high

temperatures and chemical resistance without sacrificing holding power. Generally, these

tapes are constructed with an equally heat and chemical resistant backing such as polyimide,

polyester, or fluoropolymer. When combined with silicone PSA, these tapes provide premium

properties.

Type of Tape

Characteristics

Ideal for joining silicone treated papers, fabri

c

s, and films. Often used for splicing l

o

w

Splicing Tape

energy films. Fast adhesion with minimal

pressure.

E

asy tape un

w

ind. Low static,

curl,

and

handling

issues.

High

resistance

to

shear

and

peel

forces

in

high

temperature ovens.

T

ape

acts

to

insulate,

anchor,

band,

identify,

mechani

c

ally

protect

and

cushion.

It

also provides a barrier to moisture and contaminants. P

redominantly use glass cloth

E

lectrical Tape

and polyester, polyimide, and fluoropolymer film as backing. T

ape must have good

electrical properties (dielectri

c

strength, arc and corona resistance, low dissipation

factor), low outgassing properties, and durability.

E

lectronic

Assembly

Tape

Plasma

Flame

T

ape must have resistance to high temperatures and chemicals. Applications include

masking during plating, w

ave soldering, chemical stripping, and conformal coating.

T

ape must have good conformability to prevent leakage.

/

T

ape must have very high temperature resi

s

tance (315°

C) and resistance to high

Spray

velocity parti

c

les. T

ape must provide clean removal without residue, conformability,

Masking Tape

and easy cutting for masking of irregular shapes.

Table 3:

Conventional T

ape Constructions Made w

ith Silicone P

SA

Silicone

pre

ssure sensitive

adhesives have also achieved success in many lamination

applications especially for electrical applications. They have been used for lamination of

flexible printed circuits and flexible flat cables where high temperature capability, adhesion to

polymer films, and electrical properties are of importance. Silicone adhesives are especially

valuable when one of the substrates is a low energy film such as polyester or fluoropolymer.

Peel strength of silicone PSA to various substrates is shown in Table 4.

Substrate

HDPE

UHMWPE

PP

PVC

Polycarbonate

Acrylic

Nylon

Stainless Steel

Immediate 2 days 7 days 14 days

190

150

185

130

310

285

295

300

220

190

295

170

310

290

315

315

245

180

283

190

290

265

290

305

285

205

290

195

295

265

305

365

Table 4:

P

eel S

trength (180°

) of Silicone P

SA on Various Substrates (Based on 1.5 mil of

adhesive

cured

w

ith 2% benzoyl peroxide on 1-mil polyimide film backing)

2

Highly crosslinked silicone

adhesive

are used in bonding of silicone rubber to metal,

fabrication of lightweight building structures and bonding of dissimilar materials. Because of

their low flammability and outgassing properties, they are also used in aerospace and

aircraft structures. Because of their hypoallergenic character, they are often used in medical

applications, such as

pressure sensitive

bandages.

Raw Materials

Silicone PSAs are comprised of two major components: a flexible, elastic silicone gum (either

all methyl based or phenyl modified) and a hard, crystalline siloxane resin. The gum is a

silanol

containing high molecular weight polydimethylsiloxane (PDMS) or

polydimethyldiphenylsiloxane (PDMDPS), and this is usually referred to as the

segment in the formulation. The

(CH

3

)

3

SiO

1/2

, and Q, SiO

4/2

, units; it is often referred to as an MQ resin. (See Figure 1)

Figure 1:

Condensation reaction of silicone

pressure sensitive adhesive.

Resin is

siloxane copolymer often referred to as MQ. P

olymer has methyl (CH

3

), as sho

wn, or

phenyl (P

h

2

SiO) groups.

2

The properties of the

adhesive

will vary with the concentrations of these two basic ingredients.

The ratio of the resin to polymer is the most important formulation detail when trying to

optimize the balance of performance properties with a given

adhesive.

Figure 2 shows the

affect of different resin / polymer ratios on tack, peel, and shear.