Silane-crosslinking efficiency in wood-polyethylene composites:

Göran Grubbström and Kristiina Oksman

Division of Design and Manufacturing of Wood and Bionanocomposites,Luleå University of Technology, Sweden

Study of different polyethylenes

10th International Conference on Wood & Biofiber Plastic Composites

Outline

Conclusions

Background

ProcessingDegree of crosslinkingMechanical properties

Testing and Results

MaterialsReactive extrusionCuring

Materials and Processing

Background

Challenges for WPCs

Improving long-term material properties

Strength and toughness

Lowering the weight

Challenges for WPCs

Improving long-term material properties

Crosslinking:

Strength and toughness

Lowering the weight

Higher strength and toughness

Creep resistance

Crosslinked WPCs

Crosslinked

Polymer matrix

Non-crosslinked

Crosslinked WPCs

Crosslinked

Polymer matrix

Non-crosslinked

Creep resistance

Crosslinked WPCs

Crosslinked

Polymer matrix

Non-crosslinked

Strength and toughness

Previous studies

Sapieha et al. (1990), Kuan et al. (2004), Xiong et al (2008), etc.Pre-treated wood flour/fibers

Add reactants during compoundingNogellova et al. (1998), Janigova et al. (2001), Bengtsson et al. (2006)

Previous studies

Sapieha et al. (1990), Kuan et al. (2004), Xiong et al (2008), etc.Pre-treated wood flour/fibers

Add reactants during compoundingNogellova et al. (1998), Janigova et al. (2001), Bengtsson et al. (2006)

Silane-crosslinkingPeroxide-crosslinking

Crosslinked polyethylene

Peroxide-crosslinking

PE

Crosslinked polyethylene

Peroxide-crosslinking Silane-crosslinking

PE

Crosslinked polyethylene

Peroxide-crosslinking Silane-crosslinking

PE

Crosslinked polyethylene

Peroxide-crosslinking Silane-crosslinking

PE

Molten state

Solid state

Crosslinked polyethylene

Peroxide-crosslinking Silane-crosslinking

PE

Side-reactio

n

(Scorch)

Molten state

Solid state

Silane-crosslinked polyethylene

Differences between type of polyethylene

HDPE LDPE

More

Faster

More

Lazar et al. (1990), Wong and Varrall (1994), Shieh and Liu (1999).

Slower

Less

Less

Curing

Polymer chain scission

Scorch

High-density polyethylene Low-density polyethylene

Silane crosslinked WPC

One-step process: CompoundingSilane-graftingProfiling

(Bengtsson et al., 2006)

Silane crosslinked WPC

Store composite

One-step process: CompoundingSilane-graftingProfiling

(Bengtsson et al., 2006)

H2O

Objective for the study

Compare results to our previous

study of crosslinked HDPE-WPC

Investigate possibility to use

silane-technology for LDPE-WPCs

Materials and Processing

Materials

Wood flourSoftwood, 300-500μm

Lubricant TPW113 (Struktol, U.S.A.)

47 wt-%

50 wt-%

3 wt-%

MFI 0.4 MFI 12HDPE LDPE (recycled)

Materials

Wood flourSoftwood, 300-500μm

Lubricant TPW113 (Struktol, U.S.A.)

VinyltrimethoxysilaneVTMS 97% (Sigma Aldrich, U.S.A.)

Dicumyl peroxide DCP 98% (Sigma Aldrich, Japan)

47 wt-%

50 wt-%

3 wt-%

Solution

MFI 0.4 MFI 12HDPE LDPE (recycled)

Materials

Wood flourSoftwood, 300-500μm

Lubricant TPW113 (Struktol, U.S.A.)

VinyltrimethoxysilaneVTMS 97% (Sigma Aldrich, U.S.A.)

Dicumyl peroxide DCP 98% (Sigma Aldrich, Japan)

47 wt-%

50 wt-%

3 wt-%

+ 4 wt-% 12:1 w/w

MFI 0.4 MFI 12HDPE LDPE (recycled)

Materials

Wood flourSoftwood, 300-500μm

Lubricant TPW113 (Struktol, U.S.A.)

VinyltrimethoxysilaneVTMS 97% (Sigma Aldrich, U.S.A.)

Dicumyl peroxide DCP 98% (Sigma Aldrich, Japan)

47 wt-%

50 wt-%

3 wt-%

+ 3 wt-% 12:1 w/w

MFI 0.4 MFI 12HDPE LDPE (recycled)

Materials

Wood flourSoftwood, 300-500μm

Lubricant TPW113 (Struktol, U.S.A.)

VinyltrimethoxysilaneVTMS 97% (Sigma Aldrich, U.S.A.)

Dicumyl peroxide DCP 98% (Sigma Aldrich, Japan)

47 wt-%

50 wt-%

3 wt-%

+ 3 wt-% 12:1 w/w

MFI 0.4 MFI 12HDPE LDPE (recycled)

25:1 w/w

Reactive extrusion

Twin screw extrusion

(Coperion W&P ZSK18 MEGAlab)

- Compounding and profiling

Throughput 5.2 kg/hResidence time 55-60 s.Melt temp. ~195oC

Curing conditions

Room temperature (RT)

Sauna (SA)90˚CRH close to 100%

21˚CRH 30-40%

Storing times: 0, 3, 6, 12 hours,1, 2, 3, 4, 6, 9 days

Sample coding

LDPE-composites HDPE-composites

HD-X LD-X

HD Non-X LD Non-X

LD-X (low)

Testing and Results

Processing

LDPE-compositeHDPE-composite

Non HD-X Non LD-X LD-X(low)

Processing

Extruder torque: 45% 60-70%35% 60%

LDPE-compositeHDPE-composite

Non HD-X Non LD-X LD-X(low)

Processing

Extruder torque: 45% 60-70%35% 60%

Die swelling: 4% 20% 40%Not significant

LDPE-compositeHDPE-composite

Non HD-X Non LD-X LD-X(low)

Degree of crosslinking

(ASTM D2765)Measure insoluble gel content

- Sample in 120 Mesh pouch- In boiling xylene for 12 hours- Extracted mass (%) measured- Degree of crosslinking = 100 – Extract (%)

Degree of crosslinking

RTSA

0%

20%

40%

60%

80%

100%

0 1 2 3 4 5 6 7 8 9

LD-X

56%

Storing time (days)

HD-X

0%

20%

40%

60%

80%

100%

0 1 2 3 4 5 6 7 8 9

35%

Storing time (days)

Degree of crosslinking

RTSA

LD-X (Low)

0%

20%

40%

60%

80%

100%

0 1 2 3 4 5 6 7 8 9

39%

Storing time (days)

56%

0%

20%

40%

60%

80%

100%

0 1 2 3 4 5 6 7 8 9

HD-X

Storing time (days)

35%

Mechanical testing

LDPE-compositesFlexural properties (ISO 178)

HDPE-composites*Tensile properties (ASTM D638)

* Grubbström and Oksman 2009

Mechanical properties

HDPE-composites

0

5

10

15

20

25

Tens

ile s

tres

s (M

Pa)

RTSA

HD-X

Non-X

Strain (%)0 1 2 3 4 5

Mechanical properties

0

5

10

15

20

25

30

Flex

ural

str

ess

(MPa

)

Non-X LD-X

LD-X (low)

Strain (%)

LDPE-compositesRTSA

0 2 3 4 5 6 7 8 91

Morphology

LDPE-Composite

HDPE-Composite

CrosslinkedNon-X

Short-term creep

DMA

- Specimen: 60.0 x 12.5 x 2.5 (mm)

- Dual cantilever mode- Static stress 5 MPa- 30˚C - 5 hours + 1 hour recovery

(TA Instruments)

Short-term creep

LDPE-composites

0,0

1,0

2,0

3,0

4,0

5,0

0 1 2 3 4 5 6

Non-XLD-X

LD-X (low)

Time (hours)

0,0

0,5

1,0

1,5

2,0

2,5

3,0

0 1 2 3 4 5 6

HDPE-composites

Non-X

HD-X

Time (hours)

RTSA

Stra

in(%

)

Conclusions

Conclusions

There are differences in crosslinking efficiencydepending on type of polyethylene in the WPC

LDPE-composite: More sensitive for reactants

Cures faster

Do not need storing in Sauna

The technology works for both HDPE and LDPE

Future work

Minimize crosslinking in extrusion process

Silane-crosslinking process: RT, SA

Long-term properties

Acknowledgements

Skellefteå Kraft and Nordea for financial support

Questions?