WOOD PLASTIC COMPOSITES: SURFACE MODIFICATION WITH

HEAT TO IMPROVE THE ADHESION ABILITY

Dr. Athanasios Dimitriou

Dr. Mike D Hale

Dr. Morwenna Spear

INTRODUCTION

� What are Wood Plastic Composites (WPCs)?� WPCs are composed of lignocellulosic fibres (e.g. wood

fibres) and thermoplastic polymers (e.g. polyolefins) in varying percentages (40-80% plastic)

� Polymers which are used for WPC production

� Low and high density polyethylene (LDPE, HDPE)

� Polypropylene (PP)

� Polyvinyl chloride (PVC) and

� Polystyrene (PS)

� Polyolefins like PP and PE are ideal for WPCs because they are safe and have low melting points (good for wood fibre)

� WPC production methods are the extrusion, injection moulding and compression moulding

INTRODUCTION -EXTRUSION LINE PRODUCTION OF WPC

+

Wood flour

Extruder

Extruded profile

Cooling stage

Trimming stage Final product length cut

Plastic granules

mixing

INTRODUCTION

� Products and uses

� Decking

� Railing

� Fencing

� Roofing

� Picnic tables and industrial flooring

� Windows and doors

� Automotive

� Furniture

INTRODUCTION

� Advantages� Durability

� Minimum moisture absorbance – more dimensionally stable than wood

� Low maintenance

� Can use recycled plastic and wood

� Therefore very environmental friendly

� Profiles and complicated forms can be produced from moulds

� Disadvantages� Thermal stability in very hot

climates

� Surface cracking in wet and subzero temperatures

� Fire resistance poor with prolonged exposure to flame

� Poor adhesion between pieces of WPC when forming a joint

INTRODUCTION–IMPORTANT ADHESION FACTORS

� Mechanical factor is that the adhesion strength can be increased by roughness of the material, resulting in an anchoring of the adhesive in the material surface.

� The thermodynamic theory is based on the interface forces such as van der Waals and Lewis acid-base interaction forces. Those forces can be related to surface free energy and the wettability because the adhered joint is produced by a solid-liquid contact stage

� The chemical bond theory is the formation of chemical bonds between the material and the adhesive. Those bonds are considered to be the strongest among the rest of the adhesion theories

INTRODUCTION –POOR ADHESION?

� Poor adhesion problem, why?

� The polyolefins have low surface energy and lack of

wettability and thus form weak bonds

� Solutions for the poor adhesion of polymers

� Treatments can be applied to the surfaces to overcome

these problems including flame, corona discharge, plasma,

UV irradiation, chromic acid treatment, fluorination,

microwave

� Surface energy increase seen by surface modification is in

most cases due to oxidation processes

INTRODUCTION -IDEAL SOLUTIONS?

� Solutions to these problems

� May use expensive equipment

� Sophisticated procedures

� Simple proposed methods were therefore selected:

� Flame

� Hot air gun

� Halogen heating lamps

� Easy to apply techniques in a small workshop

MATERIALS

� WPC = 60% spruce flour, 35.5% polypropylene, 2% coupling

agent, 1.5% UV-stabilizer and 1% colour pigment

� Compared epoxy, polyurethane and PVA adhesives

� A two part commercial epoxy adhesive was chosen

METHODS-SURFACE TREATMENT

� Surface modification methods used

� Hot air gun treatment at different levels of exposure

by moving gun over the surface at different

speeds* from 18.5 to 115 and 2 times with the

speed of 75

� Flame treatment, at different speeds* of 125 to

250 in steps of 25

� Halogen heating lamps treatment, at speeds* of 10

to 50 in steps of 10

*in mm s-1

METHODS-SURFACE TREATMENT

Hot air gun treatment Halogen heating lamps

treatment

MATERIALS AND METHODS –ADHESION BOND STRENGTH

ASSESSED

� Adhesion shear strength determination, EN 205

� Lap joint shear test (20 samples per variable)

� Samples with failed in the material were excluded

Adhesion strength evaluation of sample Strength test instrument

MATERIALS AND METHODS, -OTHER FEATURES ANALYSED

Surface Characterisation

� Surface roughness determination

� Surface energy determination (wettability)

� FTIR-ATR chemical analysis

� Scanning electron microscopy (SEM)

Surface roughness Contact angle /surface energy

RESULTS- HOT AIR GUN TREATMENT EXAMPLE

350

450

550

650

750

850

15 35 55 75 95 115 135 155

N

Treatment speed (mm s-1)

control mean

SD

2X75 mm s-1

RESULTS-ADHESION STRENGTH

350400450500550600650700750800850900

15 35 55 75 95 115 135 155

N

mm s-1

350400450500550600650700750800850900

100 125 150 175 200 225 250 275

N

mm s-1

350

400

450

500

550

600

650

0 10 20 30 40 50 60

N

mm s-1

b: Hot air gun treatmenta: Flame treatment

c: Halogen heating lamps

2X75

RESULTS-OPTIMUM TREATMENTS

0

20

40

60

80

Control Hot air treatment (2X75 mm s⁻¹)

Hot air treatment (115 mm s⁻¹)

Flame treatment (175 mm s⁻¹)

Halogen heating lamps treatment

(30 mm s⁻¹)

450

550

650

750

850

Ma

teri

al

fail

ure

(%

)

Sh

ea

r str

en

gth

(N

)

Shear strength mean in N Material failure %

RESULTS-SURFACE ROUGHNESS

-40%

-20%

0%

20%

40%

60%

80%

100%

120%

140%

160%

% c

ha

ng

e r

ela

tive

to

SR

of

co

ntr

ol

Hot air treatment

(2X75 mm s-1)

Hot air treatment

(115 mm s-1)

Flame treatment

(175 mm s-1)

Halogen heating lamps treatment

(30 mm s-1)

RESULTS-SEM

b: Hot air gun treatmenta: Flame treatment

c: Halogen heating lamps

RESULTS-SURFACE ENERGY

Hydrogen peroxide treatment (pH 7.5)

Hot air treatment

(2X75 mm s-1)

Hot air treatment

(115 mm s-1)

Flame treatment

(175 mm s-1)

Halogen heating lamps treatment

(30 mm s-1)

-10%

0%

10%

20%

30%

40%

50%

60%

70%

% c

ha

ng

e r

ela

tive

to

SE

of

co

ntr

ol

Hot air treatment

(2X75 mm s-1)

Hot air treatment

(115 mm s-1)

Flame treatment

(175 mm s-1)

Halogen heating lamps treatment

(30 mm s-1)

RESULTS-FTIR ATR

HT18.5_Average

80

85

90

95%

R

HT2X75_Average

80

85

90

95

%R

control_Average

80

85

90

95

%R

500 1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm-1)

esters

Ratio: Ar

RESULTS-FTIR ATR

-10%

-5%

0%

5%

10%

15%

20%

25%

30%

35%

40%

1732 Ester Carbonyl

Ar%

Hot air treatment

18.5 mm s⁻¹

25 mm s⁻¹

31 mm s⁻¹

37.5 mm s⁻¹

67.5 mm s⁻¹

75 mm s⁻¹

2X75 mm s⁻¹

115 mm s⁻¹

-10%

-5%

0%

5%

10%

15%

20%

25%

30%

35%

40%

1732 Ester Carbonyl

Ar%

Flame treatment

125 mm s‾¹

150 mm s‾¹

175 mm s‾¹

200 mm s‾¹

225 mm s‾¹

250 mm s‾¹

-10%

-5%

0%

5%

10%

15%

20%

25%

30%

35%

40%

1732 Ester Carbonyl

Ar%

Halogen heating lamps treatment

10 mm s‾¹

20 mm s‾¹

30 mm s‾¹

40 mm s‾¹

50 mm s‾¹

DISCUSSION

� Overall impact of the treatment methods to the roughness

of the WPC surface and its relations to the adhesion effect

Grey colour (�) Control, (�) flame treatment, (�) hot air gun treatment

and (�) halogen heating lamps treatment

DISCUSSION

� Overall impact of the treatment methods to the surface

energy of the WPC and its relations to the adhesion effect

Grey colour (�) Control, (�) flame treatment, (�) hot air gun treatment

and (�) halogen heating lamps treatment

DISCUSSION

� Overall impact of the treatment methods to the carbonyl

content of the WPC surface and its relations to the

adhesion effect

Grey colour (�) Control, (�)flame treatment, (�) hot air gun treatment

and (�) halogen heating lamps treatment

CONCLUSION-PRACTICAL CONSIDERATIONS

� Hot air treatment� Equipment needed:

� Hot air gun

� Advantages:� Highest adhesion strength

� Do not seem to cause any damage the surface

� Disadvantages:� Very accurate procedure.

Slowest treatment speed caused adhesion strength decrease

� Interesting points:� Two times pass treatment

provided the best bond

� Flame treatment� Equipment needed:

� Flame torch with a MAPP gas tank

� Advantages:� Quickest method

� Disadvantages:� Hazardous procedure

� Very accurate procedure

� Degradation on the material surface

CONCLUSION-PRACTICAL CONSIDERATIONS

� Halogen heating lamps

� The treatment caused adhesion strength decrease

� Could be used as a treatment method to reduce the

adhesion ability of polymer materials, for surface protection

from paints and adhesives, in uses like workbenches

� Although the halogen treatment increased carbonyl content,

surface energy and surface roughness to a small extent the

adhesion strength decreased

� Further investigation of the treatment could be interesting to

understand the reasons behind this less effective adhesion

CONCLUSIONS

Flame Hot air Halogen

Mechanical Clear change in

roughness – pitting

and prominent

particles

Slight reduction in

surface roughness

No Significant

change

Surface energy Clear change in

surface energy

No Significant

change

Clear change in

surface energy

Chemical Clear change in

ester formation

Clear change in

ester formation

Clear change in

ester formation

ATHANASIOS DIMITRIOU

Thank you for your attention!