Introduction to Pressure Sensitive Adhesives:

Polymer Types

ASC Short Course

April 28, 2014

Werner A. Hoermann

What does a customer expect from a PSA?

• Adhesion on demand

• at ambient temperatures

•without the need for sophisticated equipment

• and without a steep learning curve

The Basics

• In an environment of insecure supply you need to be flexible

• Changes in user industries drive PSA development

• The adhesive is always at fault if something goes wrong

• You can’t overestimate the ability of the user to screw up

• There is no limit to the number of possible PSA formulations

and ingredients

Requests for PSA development

•Make it cheaper

•We can’t get that raw material anymore

• The customer wants more …(peel, shear, etc.)

•We need a knockoff of the competitors product X

•We have this great new product idea

Design Considerations for a PSA

• What surfaces to bond to

• What carrier (paper - label, fabric – band aid, plastic - membrane, differential release liner, etc.)

• What are the application conditions of the finished product (temperatures, time, pressure, cleanliness,

applicator sophistication)

• What are the lifetime bond stress conditions (temperatures, peel, shear, tack)

• What are the environmental stresses (uv, ozone, oxygen, plasticizers, moisture, mold, wind, etc)

• Environmental and Health considerations

• What production equipment is available

• What application (coating) equipment is available

• What raw materials are available at what price

Compounding and Coating Methods

• Compounding• Solvent based

• Water based

• 100% solids

• Coating Methods• Solvent based

• Water based

• 100% solids

• On the web x-linking

How does the PSA bonding process work?

Best Case: Excellent wet-out, plenty of contact, high surface energy

Worst Case: Bad wet-out, little contact, low surface energy

PSA Performance as a Function of Temperature

-50.0 -25.0 0 25.0 50.0 75.0 100.0 125.0 150.0

temperature (°C)

100.0

1000

10000

1.000E5

1.000E6

1.000E7

1.000E8

1.000E9

G' (P

a)

100.0

1000

10000

1.000E5

1.000E6

1.000E7

1.000E8

1.000E9

G''

(Pa)

0

0.2500

0.5000

0.7500

1.000

1.250

1.500

1.750

2.000

2.250

2.500

|tan(d

elta

)|

Temperature Sweep, 10 rad/s

Butyl Hybrid

tan delta peak

Glass Region

Transition Region

Rubber Plateau

Melt Flow

G'

G"

Softening Point

Dahlquist Criterion - 300,000 Pa

tan delta minimum

tan delta = G"/G'

What are the basic PSA material properties?

• Polymeric – above entanglement molecular weight

• Substantially amorphous (i.e. not crystalline)

• Glass transition temperature (Tg) well below room temperature

• Low modulus under bonding conditions (temperature and time

scale)

• Relatively low surface energy

PSA Polymer types

• Hydrocarbon Elastomer Based

• Acrylic Copolymers

• Silicone Based

• Polyolefins

• Polyvinyl ethers

Components of Rubber/Resin PSAs

• Polymer

High molecular weight elastomer with low Tg

• Resin

Low molecular weight oligomer with Tg typically higher than the elastomer

• Plasticizer

Low molecular weight liquid with low Tg

Elastomers for PSAs

• Natural Rubber

• Styrene-butadiene random copolymer

• Polybutadiene

• Polyisoprene

• Polyisobutylene/Butyl

• Styrene block copolymers

Elastomers for PSA

Polyisoprene (cis- 1,4)

natural rubber

synthetic PI (e.g. Natsyn®)

CH2 CH2

C C

H CH3

n

Elastomers for PSA

styrene-butadiene rubber (SBR)

butadiene-acrylonitrile (nitrile rubber)

Polybutadiene and copolymers

cis 1,4 polybutadiene

CH2 CH 2

C C

Elastomers for PSA

Polyisobutylene

butyl rubber (isobutylene and smallamount of isoprene copolymerized)

CH3

CH2 C

CH3

n

SBC Architecture

star

triblock

diblock

usually styrene-isoprene orstyrene-butadienebased

The SBC network

Physical

Crosslinks

Polystyrene Domains

(T9

95° C)

Rubber

Phase

,,/"

Rubber Phase

--- Polybutadiene (T9 -85° C)

Polyisoprene (T9 -60° C)

Polyolefin (T9 -55° C)

SBC type choice and effect

Effect of Resin on SBC

• Raises Tg of blend compared to rubber alone

• Reduces plateau modulus, acts as diluent of entanglements

once you are in the rubbery zone

• Combination of these two effects allows the system to meet

the Dahlquist criterion for tack and the requirement for high

energy loss in debonding time scale

• Tackifier must be thermodynamically miscible with polymer

Effect of Plasticizers, Oils, Liquid Resins

dilutes rubber and contributes to reduction in plateau modulus

effects a lower composite Tg than using solid

tackifier alone

General Properties of Rubber Resin Systems

• Availability of wide array of polymers, resins, plasticizers and fillers

• Can be formulated to be inexpensive

• Can be formulated to stick to a wide variety of surfaces, even non-

polar ones

• Sensitivity to light, oxygen, weathering and plasticizers, depends

upon choice of grade

• Suitable for all types of compounding and coating systems

Acrylic Copolymers

• Low Tg Monomers• 2-ethyl hexyl acrylate• butyl acrylate• iso-octyl acrylate

• High Tg monomer(s)• methyl acrylate• methyl methacrylate

• Polar monomers (typically also high Tg)• acrylic acid• Acrylamide• n-vinyl pyrrolidone

Typical Properties of Acrylic PSAs

• Moderate cost

• Adhesion varies significantly with substrate chemistry

• Very stable to oxidation, weathering

• Wide range of performance achieved through copolymerization chemistry

• High adhesion build over time to certain surfaces

• Usually no need for tackifiers

• Available for WB, SB, and 100% solid technologies

Silicone Pressure Sensitive Adhesives

• Silicone polymer/gum + Silicate resin• Partially coreacted by condensation of terminal

silanol groups on gum and resin• Evidence for two phase structure:

continuous gum-rich phasediscontinuous resin-rich phase

• Gum is low Tg, resin is high Tg

Typical properties of Silicone PSAs

• Expensive

• Sticks to virtually anything - can even bond underwater and

stay bonded on glass when wet

• Very stable to light, heat, oxygen

• Limited range of performance with formulation

• Very wide temperature range

Other Polymers for PSA’s

• high vinyl acetate EVA copolymers plus tackifiers

• atactic polypropylene

• polyvinyl ethers and copolymers with acrylics

• polyurethane

Final WordsStay flexible and open minded – There will be constantly new materials and technologies for you to work with

Think through the demands on the adhesive throughout the life cycle of the finished product

- from ingredient sourcing - to compounding - to coating - to storage - to application - to in place performance - to disposal

Formulation is only one aspect of finding a good PSA solution