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