48th Annual Convention
Society of Wood Science and Technology
New Protection Systems With
Silicones and Silanes
C. Mai, S. Donath, O. Weigenand, H. Militz
Institute of Wood Biology and Wood Technology
University of Göttingen
General Aspects
• Si is the second most frequent element on
earth
– Makes up 27.5% of the earth‘s crust
– Is predominantly found as polymeric SiO2 :
m SiO2.n H2O - [Si(OH)4]
• Si occurs in higher concentration in
tropical wood species
Institute of Wood Biology and Wood Technology
University of Göttingen
General Aspects of Silicon
Natural process of silification:
Petrified wood
From: Museum of natural history, Vienna
Institute of Wood Biology and Wood Technology
University of Göttingen
Chemistry and Nomenclature
• Silanes: Hydrogen compounds of silicon
SinH2n+2 (n=1-15)
H
Si
H
H
Monosilane
H
H
H
H
Si
Si
H
H
H
Disilane
• Halogen silanes: SiH3Cl; SiH2Cl2, SiHCl3
• Silicic acid:
OH
HO
Si
OH
Institute of Wood Biology and Wood Technology
University of Göttingen
OH
n S iO 2 + 2n H 2 O
Silicate
Chemistry and Nomenclature of Silicon Compounds
• Silanol: H3SiOH Silanediol: H2Si(OH)2 etc.
H
n HO
H
Si
OH
-n H2O
HO
H
O
Si
Si
H
H
O Si
etc.
H
Siloxane
• Silicone (polyalkylsiloxane): Organic derivatives of
poly(silicic acid)
R
R
Si
R
R
R
O
Si
O
R
Si
R
R
O
Si
etc.
R
– Si-C bond is stable towards acids and bases
Institute of Wood Biology and Wood Technology
University of Göttingen
R = Alkyl
Aims of Silicon-Treatment
1. Protection against degrading and staining fungi
and insects
¾
Basidionmycetes / Moulds
¾
No biocide
2. Improvement of properties related to moisture and
weathering
¾
Swelling /shrinkage
¾
adhesion of coatings and varnish
¾
UV degradation
à Incorporation of silicon compounds into the cell
wall
Institute of Wood Biology and Wood Technology
University of Göttingen
Aims of Silicon-Treatment
3. Improvement of elasto-mechanical properties
¾ hardness
¾ tensile strength
¾ fire resistance
à Can be achieved by deposition of silicon
compounds into the lumina of the wood cells
Institute of Wood Biology and Wood Technology
University of Göttingen
Treatments
• Inorganic silicates (“water
glass”)
• Silanes (sol-gel process)
• Silicones
• Micro-emulsion technology
Institute of Wood Biology and Wood Technology
University of Göttingen
“Water glass”
What is “water glass”?
• Water-soluble K or Na-silicates or their solutions
• Alkaline (pH over 12)
• Problem: Fixation:
– Precipitation in the wood through acids
– Precipitation with salts (CaCl2, Al2(SO4)3, borax)
Institute of Wood Biology and Wood Technology
University of Göttingen
“Water glass”
Fixation:
Matthes et al., Erfurt (1998 – 2003)
– Precipitation through CO2 from air
– Needs several weeks for curing
Furuno et al. (1991 - 2001)
– Precipitation through salts (2 impregnation steps)
• E.g. boron compounds, Al2(SO4)3 or CaCl2, BaCl2
Institute of Wood Biology and Wood Technology
University of Göttingen
Penetration of water glass
Penetration was determined by means of
SEM-EDX
• x-ray mapping of Si
Si
0
Institute of Wood Biology and Wood Technology
University of Göttingen
5
10
„Water glass“: white / brown rot
• Precipitation of silicate within wood by
gaseous acetic acid
• Biocidal leachable compounds are
formed
Institute of Wood Biology and Wood Technology
University of Göttingen
„Water glass“: white / brown rot
T. versicolor (Beech)
C. puteana (Pine)
70
Weight loss [%]
60
70
10%
20%
30%
10%
60
30%
50
50
40
40
30
30
20
20
10
10
0
0
neutralized untreated (n)
alkaline
untreated (a)
Institute of Wood Biology and Wood Technology
University of Göttingen
20%
neutralized untreated (n)
alkaline
untreated (a)
“Water glass”
Results
• Decay resistance (EN 113)
– High
• Because of high pH
•
•
•
•
Enhanced fire resistance
Increased hygroscopicity
High pH (problems at high drying temperature)
Mechanical
–
–
–
–
ASE low increase
Hardness unchanged or increased
Strength is significantly reduced
Dynamic MoE un-changed
Institute of Wood Biology and Wood Technology
University of Göttingen
“Water glass”
MASID umwelterhaltende Produkte GmbH
• “Woodbliss”: (aqueous solution, pH >12)
– More than 10% silicate (water glass),
up to 10% silicic acid (to improve penetration)
Timbersil (Springfield, VA)
• „Sodium Silicate Technology (SST)“
Institute of Wood Biology and Wood Technology
University of Göttingen
Treatments
• Inorganic silicates (“water glass”)
• Silanes (sol-gel process)
• Silicones
• Micro-emulsion technology
Institute of Wood Biology and Wood Technology
University of Göttingen
Wood-inorganic composites by sol-gel process
• Two stage process
– Hydrolysis (here Silicic Acid Esters)
O Et
OE t
Tetraethoxy
silane (TEOS)
EtO
Si
OEt
+ 2 H2O
HO
- 2 EtOH
OEt
Si
OH
OEt
– Condensation
OEt
2 HO
Si
OEt
OH
- H2O
HO
OEt
Si
OEt
OEt
O
Si
OH
OEt
• End product is an amorphous silicate (glass)
Institute of Wood Biology and Wood Technology
University of Göttingen
etc.
+ H2O
..... (SiO2)n
Silicate
Silane Types
Y = “Organo-functional group”
OX = “Silicon-functional group, OCH3, OC2H5 etc.
etc
OX
Y
TEOS
SiO2
Institute of Wood Biology and Wood Technology
University of Göttingen
Si
OX
OX
MTES
Me-SiO3/2
Wood-inorganic composites by sol-gel process
Saka et al. (1992 - 2001)
How to treat?
Si must incorporated into the cell wall!!
• Pre-conditioning of wood to fibre saturation point
– Impregnation with TEOS in ethanol (free of water)
– Condensation in the cell wall
• ASE: about 42%
• Water uptake is reduced
Institute of Wood Biology and Wood Technology
University of Göttingen
Wood-inorganic composites by sol-gel process
Böcker et al. (2001), Böttcher et al. (1999)
Silanes are hydrolysed before the impregnation.
Impregnation is performed with pre-condensed
sols (in ethanol)
• Properties:
– WPG: 5 – 50%
– Reduced moisture content
– ASE: 60% (at 45% WPG)
– Low decay resistance
• Enhanced by boron addition (“controlled release”)
Institute of Wood Biology and Wood Technology
University of Göttingen
Alkoxysilanes
OEt
EtO Si OEt
CH3
EtO Si OEt
OEt
C3H7
EtO Si OEt
OEt
OEt
TEOS
MTEOS
PTEOS
Tetraethoxysilane
MethylTriethoxysilane
PropylTriethoxysilane
Institute of Wood Biology and Wood Technology
University of Göttingen
Impregnation Methods
“Sol impregnation”
“Silane impregnation”
1. Conditioning of Wood
Samples at 65% r.H.
1. Conditioning of wood
samples at 100% r.H.
(fibre saturation)
2. Hydrolysis of the silane
(Pre-hydrolysis)
3. Impregnation with Sol
4. Curing at 103°C
Institute of Wood Biology and Wood Technology
University of Göttingen
2. Impregnation with
silane/catalyst mixture
3. Curing at 103°C
Wood Samples
Examination
Wood Species
Sample Size
Water Uptake,
Dimensional stabilization
Beech
25 x 25 x 10 mm
(rad x tang x long)
Soil Bed Test
(ENV 807)
Pine Sapwood
5 x 10 x 100 mm
Beech
Pine Sapwood
5 x 10 x 30 mm
Basidiomycete Test (Miniblock)
(T. versicolor, C. puteana)
Institute of Wood Biology and Wood Technology
University of Göttingen
Penetration of Silanes
Sol (pre-hydrolysed)
Institute of Wood Biology and Wood Technology
University of Göttingen
Silane (monomer)
WPG (beech)
After curing
35
After aging
W PG [%]
30
25
20
15
10
5
0
TE
OS
MT
So
l
ES
PT
So
Institute of Wood Biology and Wood Technology
University of Göttingen
l
EO
TE
So
l
OS
MT
Si
la n
e
ES
PT
Si
la n
e
EO
S il
an
e
Dimensional stabilisation (beech)
After curing
After aging
35
ASE [% ]
30
25
20
15
10
5
0
TE
OS
MT
So
l
ES
PT
So
Institute of Wood Biology and Wood Technology
University of Göttingen
l
EO
TE
So
l
OS
MT
S il
an
e
ES
PT
S il
an
EO
e
S il
an
e
Water Immersion Test
Water Uptake of Wood Composite [mg/cm³]
360
300
240
180
Control
TEOS
Sols
120
MTEOS
Silanes
60
PTEOS
TEOS
MTEOS
PTEOS
0
0
2
4
6
8
10
12
14
Immersion Time [h]
Institute of Wood Biology and Wood Technology
University of Göttingen
16
18
20
22
24
Soil Bed Test (ENv 807)
75,0
66,9
after 8 weeks
after 16 weeks
after 24 weeks
65,0
55,0
55,9
55,5
50,0
47,7
MOE loss [%]
46,7
45,8
45,0
33,5
35,0
32,2
28,5
31,3
26,3
25,6
23,8
25,0
18,3
16,7
15,0
13,6
15,0
10,4
5,0
9,8
9,6
6,9
2,3
1,3
-5,0
12,6
10,6
4,8
2,7
-2,4
TEOS
MTEOS
PTEOS
Pine SW Pine HW
Sols
Institute of Wood Biology and Wood Technology
University of Göttingen
TEOS
MTEOS
PTEOS
Pine SW Pine HW
Silanes
Basidiomycete Test (mini-block)
Mass Loss of Beech after 6 Weeks Exposure to Trametes versicolor
40,0
38,0
37,0
36,4
35,0
mass loss [%]
30,0
25,0
20,0
14,0
15,0
10,0
8,9
7,5
5,0
4,7
4,5
MTEOS
PTEOS
0,0
Control
TEOS
MTEOS
Sols (conc. 50%)
Institute of Wood Biology and Wood Technology
University of Göttingen
PTEOS
Control
TEOS
Silanes (conc. 100%)
Weathering
Artificialsides
weathering
pine wood
in 1a year
QUV
Reverse
of woodofsamples
after
(2 weeks)
of weathering
in Göttingen
control
Silane 1
Silane 2
untreated
oligomeric silane systems
Institute of Wood Biology and Wood Technology
University of Göttingen
Conclusions (Silanes)
• Good penetration of monomeric silane in pine
and beech
• Ageing during several weeks
• Slight or no reduction of moisture uptake
• Slight dimensional stabilisation
• Good hydrophobation effects with alkyl-silanes
• Alkyl-silanes improved resistance against
fungal attack only initially
Institute of Wood Biology and Wood Technology
University of Göttingen
Silanes – Two-step Processes
Covalent Fixation to the cell wall
1.
2.
Reaction with organo-silanes
Reaction with TEOS
3-Isocyanatpropyl
triethoxysilane
β-(3,4 epoxycyclohexyl)
ethyl trimethoxysilane
High ASE !
Institute of Wood Biology and Wood Technology
University of Göttingen
vinyl trimethoxy
silane
Treatments
• Inorganic silicates (“water glass”)
• Silanes (sol-gel process)
• Silicones
• Micro-emulsion technology
Institute of Wood Biology and Wood Technology
University of Göttingen
Silicones
Silicones are polydimethyl siloxanes!
A
Si
CH3
CH3
CH3
CH3
O
Si
Si
O
R
University of Göttingen
Si
O
CH3
m
Institute of Wood Biology and Wood Technology
CH3
CH3
n
A
Silicones Emulsions - Penetration
SEM-EDX: x-ray mapping of Silicon
Si
0
5
Institute of Wood Biology and Wood Technology
University of Göttingen
10
15
Dimensional stability (Pine)
65
WPG
ASE 0-20/65
ASE 0-fs
WPG and ASEvol (%)
55
45
35
25
15
5
-5
Micro 1
(30%)
Micro 1
(15%)
Institute of Wood Biology and Wood Technology
University of Göttingen
Micro 1
(7.5%)
Micro 2
(30%)
control
Silicones - Decay Fungi (12 weeks)
70
70
T. versicolor
60
60
50
50
Weight loss [%]
Weight loss [%]
C. puteana
40
30
40
30
20
20
10
10
0
0
silicone 1 silcone 2 silicone 3
control
Institute of Wood Biology and Wood Technology
University of Göttingen
silicone 1 silcone 2 silicone 3
control
Silicones - Outside Weathering
60
silicone 1
silicone 2
control
Wood moisture [%]
50
40
30
20
10
0
0
5
10
Exposure time [weeks]
Institute of Wood Biology and Wood Technology
University of Göttingen
15
20
Silicones - Outside Weathering (6 month)
Front side
untreated
treated
Institute of Wood Biology and Wood Technology
University of Göttingen
Reverse side
untreated
treated
Conclusions (Silicons)
• Penetration was dependent on the particle size
of silicone emulsions
• Micro-emulsions enhanced the dimensional
stability
• Reduction of moisture uptake and increase in
weathering stability
• Decay resistance was dependent on functional
groups on the siloxane backbone
Institute of Wood Biology and Wood Technology
University of Göttingen
Treatments
• Inorganic silicates (“water glass”)
• Silanes (sol-gel process)
• Silicones
• Micro-emulsion technology
Institute of Wood Biology and Wood Technology
University of Göttingen
WACKER SMK® Technology
Micro-emulsions of some 10 nm diameter
•
Emulsifier:
–
•
Co-Emulsifier:
–
•
N-(2-aminoethyl)-3-aminopropyl trimethoxysilane +
α,ω-dihydroxy-methylpoly siloxane +KOH
30 nm
Isooctyltriethoxy silane
[15g + 80g A + 4g propionic acid]
Hydrophobic compound:
Silane, siloxane, polysiloxane
–
Silane, siloxane
Silane, siloxane, polysiloxane
Functional polysiloxane
Good water repellent effect, but no increase of ASE
Institute of Wood Biology and Wood Technology
University of Göttingen
Conclusions (general)
• Wide variety of chemicals is available
• Various wood properties can be improved
• Improvement of decay resistance is low
• Decay resistance can be enhance by combination
with other compounds (“controlled release”)
• High water repellence
• Application in use-class III (EN 335)
Institute of Wood Biology and Wood Technology
University of Göttingen
Thank you for your
attention!
Institute of Wood Biology and Wood Technology
University of Göttingen