© Fraunhofer WKI

Development of durable, solvent-free water-based coatings for WPC sidings

Claudia Schirp, Arne SchirpFraunhofer-Institute for Wood Research

Wilhelm-Klauditz-Institute (WKI)

Braunschweig, Germany

Cornelia Bellmann, Alfredo CalvimontesLeibniz-Institute of Polymer Research

Dresden, Germany

European Coatings Conference „Façade Coatings and Plasters“

30/31 October 2013, Düsseldorf, Germany

© Fraunhofer WKI

Why use waterbased coatings?

Liquid coatings offer high flexibility extrude one WPC formulation only and add pigmentation and UV stabilization via coating

"Difficult" colours which are often avoided in extrusion can be used as coating (white, red, bright green, etc.)

Cost efficiency

Legal requirements: VOC-directive (1999), Decopaint-directive (2004)

Halogen-free products provide advantages in terms of recycling source: Werzalit

© Fraunhofer WKI

Challenges in applying water-based coatings on WPC

Adhesion of water-based coatings on hydrophobic, polyolefin-based substrates is challenging, especially without pre-treatments (flame or plasma treatment)

Little experience with long-term durability and adhesion of water-based coatings on WPC products

Influence of various WPC formulation components (type of polymer matrix, lubricant, coupling agent) and WPC surface topography on coatings adhesion is largely unknown

© Fraunhofer WKI

Experimental Design: Fraunhofer WKI formulations

Variation in WPC formulations regarding

Wood content (50%- 80%)

Type of matrix polymer (different types of PP; one type of HDPE, also in combination with LDPE)

Lubricant and coupling agent content

Processing (heating-cooling mixer and extrusion; compounding and extrusion; direct extrusion)

© Fraunhofer WKI

Experimental Design: Industrial WPC products

Producer 1: 70% woodcontent, PE matrix, light brown

Producer 2: 55% woodcontent, PP matrix, darkgrey

© Fraunhofer WKI

Experimental Design: Pre-treatments

Atmospheric plasma treatment

Device „Plasma Cat Compact 1K / 2K“ (Diplacon)

Two nozzles with working width of 20 mm

Distance of nozzles to WPC substrate about3 mm

Velocity 17,5 - 20 m / min

Mechanical Treatments

Finished with stainless-steel, plastic andcopper brush

Sanding (80 mm grid paper)

© Fraunhofer WKI

Experimental Design: Industrial Coating Systems

System 1: hybrid based on acrylate and polyurethane

One layer system: RAL 3005 (dark red), brush or spray application

System 2: based on acrylate

Two layer system: colourless primer, topcoatRAL 3005, brush or spray application

System 3: based on two componentpolyurethane

One layer: colourless, brush or spray application

© Fraunhofer WKI

Surface characterization – stereo photogrammetry

Equipment „TraceIt“ (Innowep GmbH, Germany)

Working principal „shape from shading algorithm“

Three pictures are captured under light sources which are arranged underthree different angles (0° / 120° / 240°)

Measured area x,y = 5 mm; z = 2 mm

Resolution x,y = 5 µm; z = 3 µm

© Fraunhofer WKI

Surface characterization – stereo photogrammetry

SVo (µm3/µm2) Volume of highest peak level and surface level in relation to surface area

Sdr (%) "Real" surface area (including contours) in relation to flat level surface area

heig

ht

length

© Fraunhofer WKI

Surface characterization – dynamic contact angle

Equipment „OCA XL35“ (Data Physics, Germany)

Video-supported optical processing

Determination of dynamiccontact angle using wateras wetting liquid

© Fraunhofer WKI

Surface characterization - adhesion

Cross-cut test (ISO 2409:2013)

Parallel lines are cut into substrate

Adhesion tape is added on top andpulled off

Remaining pattern is evaluated

Test performed under dry and / or wetcondition

0

1

2

3

4

5 see photo

0

5

© Fraunhofer WKI

Results

Influence of wood content on topography, wetting properties and adhesion of PE-based WPC

Effects of plasma treatment

Influence of matrix polymer type on adhesion

Detection of lubricant and MAPP on WPC surface

Effect of lubricant on coatings adhesion and WPC surface

Influence of sanding and brushing on WPC before coating application

Coating performance on WPC after artificial weathering

© Fraunhofer WKI

Influence of wood content on topography, wetting properties and adhesion of PE-based WPC

With increasing wood content:

reduction of Sdr (means smoother surface)

Increase of advancing contact angle, reduction of wetting

Good adhesion (cross-cut test result “0”) at minimum 70% wood content after plasma or flame treatment

© Fraunhofer WKI

Effects of Plasma Treatment

Change in surface topography

Reduction of contact angle (water), better wetting Advancing contact angle on PP-based WPC: 97° -> 21° Advancing contact angle on PE-based WPC: 98° -> 86°

Improvement of adhesion Cross-cut on PP-based WPC: 5 -> 0 (dry) 5 -> 1

(wet) Cross-cut on PE-based WPC: 5 -> 0 (dry) 5 -> 4 (wet)

SVo Sdr

PPPP

© Fraunhofer WKI

Effects of Plasma treatment (illustration)

Adv

. Con

tact

ang

le [°

]

No treatmentPlasma treatment

vvvvv

vvvvv

vvvvv

vvvvv

© Fraunhofer WKI

Influence of matrix polymer type on cross-cut test (variation of PP matrix)

Cross-cut test were performed on coated, PP-WPC after plasma treatment

No/minor influence of PP-structure on coating adhesion based on cross-cut results

However, significant differences in mechanical properties, water up-take and swelling of WPC were determined

PP matrix MFI* Form Dry Wet, 2 h Wet, 24 hHomopolymer 12 Powder 0 1 1

Random Copolymer 2 Powder 0 1 1Homopolymer 10 Powder 0 1 2Homopolymer 2,1 Pellets 0 1 2Homopolymer 10,5 Pellets 0 1 1

* in g/10 min (230°C / 21,6 kg)

© Fraunhofer WKI

Detection of lubricant and MAPP on WPC surface

Lubricant and MAPP can be detected using ATR-FTIR

5 % MAPP / 0 % Lub0 % MAPP / 5 % Lub5 % MAPP / 5 % Lub0 % MAPP / 0 % Lub

1779 cm-1

MAPP 1716 cm-1

Lub 720 cm-1 Lub

Tran

smis

sion

%

Wave number cm-1

© Fraunhofer WKI

Effect of lubricant on coatings adhesion and WPC surface

Matrix type CA Lub Plasma Cross-cutdry

Cross-cutwet, 2 h

27,5 % PP 2 % MAPP 0,5 % Yes 0 027,0 % PP 2 % MAPP 1,0 % Yes 0 028,0 % PE 2 % MAPE 0 % Yes 0 327,0 % PE 2 % MAPE 1,0 % Yes 0 3

Coatings adhesion not affected by lubricant

No effect of lubricant on advancing contact angle (water) and surface smoothness (Sdr)

© Fraunhofer WKI

Influence of sanding and brushing on WPC before coating application (cross-cut test, dry)

Matrix / coating No treat-ment

Plasma Stainlesssteel

Plastic Copper Sanding

PP / 2K-PU 5 1 1 1 0 1

PP / PU-acrylate 5 1 4 5 4 2

PE / 2K-PU 5 2 2 4 5 1

PE / PU-acrylate 5 3 4 5 5 2

Can a mechanical treatment substitute plasma activation?

PP-based WPC: every mechanical treatment tested equivalent to plasma activation if 2K-PU is used

PE-based WPC: sanding is superior to plasma activation for both coatings

© Fraunhofer WKI

Artificial weathering test condition (EN 927-6)

Step Function Temperature Duration Condition 1 Condensation (45 3)°C 24 h2 Subcycle Step 3+4 48 x

3 UV (60 3)°C 2.5 h 0,89 W/m²; UV A -340nm

4 Spray 0.5 h UV off

EN 927-6: "Exposure of wood coatings to artificial weathering using fluorescent UV and water"

© Fraunhofer WKI

Coating performance on WPC after artificial weathering (EN 927-6)

WPC Matrix Plasma Brushing,industry

Cross-cutdry, start

Cross-cutdry, end

Delta E

Delta gloss

WKI PE no no 5 5 1,5 -13,7

WKI PE yes no 1 1 0,7 -5,7

Coating system: PU-acrylate resin, one layer, RAL 3005 PE-based matrix: excellent adhesion after plasma treatment

© Fraunhofer WKI

Coating performance on WKI PE-based WPC after artificial weathering (EN 927-6)

No treatment

Plasma treatment

15

© Fraunhofer WKI

Coating performance on WPC after artificial weathering (EN 927-6)

WPC Matrix Plasma Brushing,industry

Cross-cutdry, start

Cross-cutdry, end

Delta E

Delta gloss

WKI PE no no 5 5 1,5 -13,7

Prod. 1 PE no no n.d. 5 4,2 0,4

Prod. 1 PE no yes 2 3 0,4 0,0

Prod. 2 PP no no 5 5 1,3 -12,1

Prod. 2 PP no yes 0 1 1,3 -0,9

Prod. 2 PP yes yes 0 1 1,1 -0,9

Coating system: PU-acrylate resin, one layer, RAL 3005

PP-based matrix: excellent adhesion after brushing, even without plasma

© Fraunhofer WKI

Coating performance on industrial PE-based WPC after artificial weathering (EN 927-6)

No treatment brushing treatment

3

5

© Fraunhofer WKI

Coating performance on industrial PP-based WPC after artificial weathering (EN 927-6)

No treatment brushing treatment

15

© Fraunhofer WKI

Conclusions

Higher wood content improves coatings adhesion; at the same time reduced wetting was observed

Coatings adhesion not affected by lubricant and by the type of polymer matrix

Plasma treatment to improve waterborne coatings adhesion is more efficient for PP-based than for PE-based WPC

WPC require treatment before coating application, but sanding and brushing might also be an equally effective alternative pre-treatment compared to flame and plasma treatment

Excellent performance of brushed or plasma/flame treated WPC after artificial weathering was achieved

Waterborne coatings offer feasible, ecological solution for improved durability and colouring of WPC sidings

© Fraunhofer WKI

On-going investigations and planned work

Long-term effectiveness of plasma and flame activation on WPC

Natural weathering test using optimized WPC and optimized coatings in different colours

Comparison to reference siding material (laminated panels, etc.)

Further characterization of WPC surfaces to determine chemical composition in order to better understand adhesion

© Fraunhofer WKI

Acknowledgements

Sandra Hofmeister, Oliver Weber, Richard Deetz, Anja Caspari

Industrial partners

Funding provided by BMWi via AiF (Arbeitsgemeinschaft Industrieller Forschungsvereinigungen e.V.), project no 411 ZBG / 1