Short Fiber Composites

• Dispersion– No clumping– Random or uni-directional– Miscibility

• Adhesion– Responsible for stress transfer

• Maximize fiber contribution

Mechanisms of Adhesion

• Adsorption• Chemisorption• Diffusion• Electrostatic• Mechanical Interlock

Adsorption

• Intimate intermolecular contact– Forces between atoms

of the substrate and resin

• Van der Waals• Hydrogen bonds• Not a covalent bond

http://www.specialchem4adhesives.com/resources/adhesionguide/index.aspx?id=theory2

Need good wetting

Contact Angle Measurement

Surface Tension

Chemisorption

• Creation of a strong chemical bond– Covalent bonding

• Sharing electrons

– Need wettability or sorption

http://www.specialchem4adhesives.com/resources/adhesionguide/index.aspx?id=theory3

Diffusion

• Inter-penetration of polymeric chains– Adhesion

between two polymers

– Commingling of polymer chains

– Need mutual diffusion

http://www.specialchem4adhesives.com/resources/adhesionguide/index.aspx?id=theory5

Electrostatic

• Development of electrostatic forces at interface– Materials transfer

electrons• Conducting materials

http://www.specialchem4adhesives.com/resources/adhesionguide/index.aspx?id=theory6

Mechanical Interlock

• Penetration of adhesive into voids and asperities of substrate– Creates an anchor

within substrate– Need wettability– Correct rheology

http://www.specialchem4adhesives.com/resources/adhesionguide/index.aspx?id=theory4

Wood Composite Adhesion• Primarily through

mechanical interlocking– Thermosets (PF, UF)

• Good adhesion• Low viscosity• Stronger affinity

– Thermoplastics• High viscosity• No affinity

• Other potential mechanisms – Adsorption– Chemisorption

Wood-Plastic Compatibility

• Polar/Non-polar Incompatibility – Oil and Water– Mixing and Dispersion

• Processing Methods (required)• Compatibilizers

Compatibilizers for WPC’s• Coupling/Dispersing

Agents– Maleic Anhydrides– Silanes– Titanates

• Coupling– Other polymers

• Most polymers are not miscible

– Fillers• Mainly inorganic

– Glass, carbon. Metal

• Thermoset Resins– Poly Diphenylmethane

diisocyanates (pMDI)

Bicerano, J. 2005

Silanes• Monomeric Silicons

– Multi-functional• Engineered for specific

composites

• Surface Treatment– Pre-treatment of fibers and

fillers

• Cross-linking mechanism– Used to promote cross-

linked system• VTMS

• For WPC’s– Research phase

Maleic Anhydrides• Polymer based CA

– Grafted with matrix polymer • MAPP, MAPE

– Developed for inorganics• Glass fiber

– Variations• MW• Maleic content

– Compared to grafted polymer

WWW.Dupont.com

• Improves surface energy– Promotes H bonding and

possibly covalent??

MA Coupled WPC’s

• Improvement in:– Mechanical

• MOR

– Physical• Water resistance

– Creep? ? ?• Relative to increased

load

Uncoupled MOE Coupled MOE Uncoupled MOR Coupled MOR

MO

E (p

si)

0

2e+5

4e+5

6e+5

8e+5

MO

R (p

si)

0

1000

2000

3000

4000

5000

6000

MA Coupled WPC’s

• Water Resistance– Lowers diffusion

coefficient– Improves durability– Less weathering

effects

Water Sorption of HDPE-WPC's

Soak TIme (hr)

0 500 1000 1500 2000 2500

Wat

er S

orpt

ion

(%)

0

5

10

15

20

25

UncoupledCoupled

Processing MA Coupled WPC’s

• MA Extrusion Influences– Can lower output– Require different

lubricants– More difficult to run

• Surface appearance

– Change in rheology• Lower melt pressures

Screw Rate (rpm's)5 10 15 20 25 30 35

Out

put (

lb/h

r)

0

200

400

600

800

1000

1200

HDPE-T1HDPE-T2 HDPE-CA

Screw RPM

5 10 15 20 25 30 35

Mel

t Pre

ssur

e (p

si)

200

400

600

800

1000

1200

HDPE T1HDPE T2HDPE CA

pMDI

• Common for traditional wood composites– Strong bond with wood

• Consume water

• Concept– Sticks to anything

• WPC’s– Limited work– Issues

• Low cure temp• Volatiles

– Foams– Needs water

• Dispersion….

References

• http://www.specialchem4adhesives.com• Bicerano, J. 2005. A practical guide to

polymeric compatibilizers for polymer blends, composites, and laminates. http://www.specialchem4polymers.com