8 Reaction of Alkylene Oxides with Wood ROGER M. R O W E L L Forest Products Laboratory, P.O. Box 5130, Madison, Wis. 53705
All of the commercial wood preservatives presently used i n the United States are e f f e c t i v e in preventing attack by microorganisms because of their toxic nature. Because of the concern these chemicals have on the environment, a l t e r n a t i v e methods based on nontoxic procedures are being investigated. Chemical modification as a possible preservative treatment for wood is based on the theory that enzymes (cellulase) must directly contact the substrate (cellulose) and this substrate must have a s p e c i f i c configuration. I f the cellulosic substrate is chemically changed, t h i s highly s e l e c t i v e reaction cannot take place. Chemical modification can also change the hydrophilic nature of wood. In some cases water, a necessity for decay organisms, is excluded from b i o l o g i c a l s i t e s . The chemicals used for modification need not be toxic to the organism because their action renders the substrate unrecognizable as a food source to support microbial growth. For wood preservation, t h i s means that it is possible to treat wood i n such a manner that attack by wooddestroying fungi will be prevented and the material will be safe for humans to handle. For wood usages i n which human contact is e s s e n t i a l , nontoxic preservatives may well be s p e c i f i e d or required i n the future. An added benefit of most chemical modification treatments to wood is the r e s u l t i n g bulking action gives the treated wood very good dimensional stability. The objective of t h i s research area is to develop a permanent, nonhazardous preservative based on the chemical r e a c t i v i t y of the wood components. Several requirements must be met for a successful treating system. Of the thousands of chemicals available, either commercially or by synthetic means, most can be eliminated because they fail to meet the properties listed below: 1. The chemical must contain functional groups which will react with hydroxyl groups of the wood components. This may seem obvious to most, but there are many literature c i t i n g s of chemicals that fail to react with wood components when, in f a c t , they
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d i d not c o n t a i n f u n c t i o n a l groups which could r e a c t . They should never have been t r i e d i n the f i r s t p l a c e . 2. The o v e r a l l t o x i c i t y o f the chemicals must be c a r e f u l l y considered. The chemicals need have no t o x i c i t y t o the microorganisms , should not be t o x i c or c a r c i n o g e n i c to humans i n the f i n i s h e d product, and should be as nontoxic as p o s s i b l e i n the t r e a t i n g stage. The chemical should be as noncorrosive as p o s s i b l e to e l i m i n a t e the b u i l d i n g o f s p e c i a l s t a i n l e s s s t e e l or g l a s s l i n e d t r e a t i n g equipment. 3. In c o n s i d e r i n g the ease with which excess reagents can be removed a f t e r treatment, a l i q u i d with a low b o i l i n g p o i n t i s advantageous. I f a gas system i s used, a low l e v e l of chemical s u b s t i t u t i o n i s u s u a l l y achieved and there are problems i n p r e s s u r i z e d gas handling. Likewise i f the b o i l i n g p o i n t i s too h i g h , i t w i l l be very d i f f i c u l t to remove a f t e r treatment. I t i s gene r a l l y true that the lowest member o f a homologous s e r i e s i s the most r e a c t i v e and w i l l have the lowest b o i l i n g p o i n t . The b o i l i n g p o i n t range t o be considered i s 30°-150° C. 4. In whole wood, a c c e s s i b i l i t y of the t r e a t i n g reagent to the r e a c t i v e chemical s i t e s i s a major c o n s i d e r a t i o n . To i n crease p e n e t r a t i o n and a c c e s s i b i l i t y , the chemical system must s w e l l the wood s t r u c t u r e . I f the reagents do not s w e l l the wood, then another chemical or co-solvent can be added to meet t h i s requirement. 5. Almost a l l chemical r e a c t i o n s r e q u i r e a c a t a l y s t . With wood as the r e a c t i n g s u b s t r a t e , strong a c i d c a t a l y s e s cannot be used as they cause extensive degradation. The most favorable c a t a l y s t from the standpoint of wood degradation i s a weakly a l k a l i n e one. The a l k a l i n e medium i s a l s o favored as i n many cases these chemicals s w e l l the wood s t r u c t u r e and give b e t t e r penetration. The p r o p e r t i e s o f the c a t a l y s t p a r a l l e l those of reagents, i . e . , low b o i l i n g p o i n t l i q u i d , nontoxic, e f f e c t i v e a t low temperatures, e t c . In most cases, the organic t e r t i a r y amines a r e best s u i t e d . 6. The experimental r e a c t i o n c o n d i t i o n s which must be met i n order f o r a given r e a c t i o n to go i s another important c o n s i d e r a t i o n . The temperature r e q u i r e d f o r complete r e a c t i o n must be low enough so there i s l i t t l e o r no wood degradation, i . e . , l e s s than 120° C. The r e a c t i o n must a l s o have a r e l a t i v e l y f a s t r a t e of r e a c t i o n with the wood components. I t i s important to get as f a s t a r e a c t i o n as p o s s i b l e at the lowest temperature without wood degradation. The moisture present i n the wood i s another c o n s i d e r a t i o n i n the r e a c t i o n c o n d i t i o n s . I t i s i m p r a c t i c a b l e to dry wood to l e s s than 1% moisture, but i t must be remembered that the OH group i n water i s more r e a c t i v e than the OH group a v a i l a b l e i n wood components, i . e . , h y d r o l y s i s i s f a s t e r than s u b s t i t u t i o n . The most f a v o r a b l e c o n d i t i o n i s a r e a c t i o n which r e q u i r e s a t r a c e of moisture and the r a t e o f h y d r o l y s i s i s r e l a t i v e l y slow.
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Another c o n s i d e r a t i o n i n t h i s area i s t o keep the r e a c t i o n simple. Avoid the multicomponent systems that w i l l r e q u i r e complex s e p a r a t i o n a f t e r r e a c t i o n f o r chemical recovery. The o p t i mum here would be i f the r e a c t i n g chemical s w e l l s the wood s t r u c ture and i s the s o l v e n t as w e l l . 7. There should be no byproducts produced during the r e a c t i o n that have t o be removed. I f there i s not a 100% reagent s k e l e t o n add-on, then the chemical cost i s higher and may r e q u i r e recovery of the byproduct f o r economical reasons. 8. The chemical bond formed between the reagent and the wood components i s o f major importance. For permanence, t h i s bond should have great s t a b i l i t y to withstand weathering. In order of s t a b i l i t y , the types o f covalent chemical bonds that may be formed are: ethers > a c e t a l s > e s t e r s . The ether bond i s s t a b l e to a c i d s and bases; the a c e t a l s t o bases but l a b i l e t o a c i d s and e s t e r s are l a b i l e to both a c i d s and bases. It i s obvious that the ether bond i s the most d e s i r a b l e covalent carbonoxygen bond that can be formed. These bonds are more s t a b l e than the g l y c o s i d i c bonds between sugar u n i t s i n the wood p o l y s a c charides so the wood polymers would degrade b e f o r e the g r a f t e d ether. A l l of these bond p o s s i b i l i t i e s consider only covalent bonding with hydroxyl groups; however, other types of chemical attachments are p o s s i b l e . For example, hydrogen bonding, i o n i c i n t e r a c t i o n s , complexing, c h e l a t i o n , and encapsulation are a l l p o s s i b i l i t i e s but l e s s permanent. 9. The hydrophobic nature of the reagent needs to be cons i d e r e d . The chemical added t o the wood must not i n c r e a s e the h y d r o p h i l i c nature of the wood components. I f the h y d r o p h i l i c i t y i s i n c r e a s e d , the s u s c e p t i b i l i t y t o micro-organism a t t a c k i n creases. The more hydrophobic the component can be made, the b e t t e r the s u b s t i t u t e d wood w i l l withstand dimensional changes i n the presence of moisture. 10. S i n g l e s i t e s u b s t i t u t i o n versus polymer formation i s another c o n s i d e r a t i o n . The g r e a t e r the degree o f chemical subs t i t u t i o n (D.S.) of wood components, the b e t t e r i t i s f o r r o t r e s i s t a n c e . So, f o r the most p a r t , a s i n g l e reagent molecule that r e a c t s with a s i n g l e hydroxyl group i s the most d e s i r a b l e . C r o s s l i n k i n g can occur when the reagent contains r e a c t i v e groups which s u b s t i t u t e two hydroxyl groups. C r o s s l i n k i n g can cause the wood to become more b r i t t l e so reagents i n t h i s c l a s s must be chosen c a r e f u l l y . Polymer formation w i t h i n the c e l l w a l l attached to wood components gives good b i o l o g i c a l r e s i s t a n c e and the b u l k i n g a c t i o n of the polymer gives the added property of dimensional s t a b i l i z a t i o n . The disadvantage of polymer formation i s that a higher l e v e l of chemical add-on i s r e q u i r e d f o r the b i o l o g i c a l r e s i s t ance than i n the s i n g l e s i t e r e a c t i o n s . 11. The t r e a t e d wood must s t i l l possess the d e s i r a b l e p r o p e r t i e s of wood. The strength must remain h i g h , no or l i t t l e
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change i n c o l o r (unless t h i s i s a d e s i r e d requirement), good e l e c t r i c a l i n s u l a t i o n , not dangerous to handle, g l u a b l e , and finishable. In t h i s study, the goal o f chemical m o d i f i c a t i o n i s to i n c r e a s e the decay r e s i s t a n c e and the dimensional s t a b i l i t y of wood. Chemical m o d i f i c a t i o n can a l s o be used to give a d d i t i o n a l improvements such as r e s i s t a n c e to c o r r o s i o n , u l t r a v i o l e t degrad a t i o n , and f i r e . 12. A f i n a l c o n s i d e r a t i o n i s , o f course, the c o s t . In the l a b o r a t o r y experimental stage, i t i s not a major f a c t o r due to the high cost of chemicals when produced on a small s c a l e . For commercialization of a chemical m o d i f i c a t i o n f o r wood, the chemic a l cost i s a very important f a c t o r . On today's market, the l i m i t of chemical cost of t r e a t e d wood f o r r o t r e s i s t a n c e cannot exceed 500 per cubic f o o t . In s p e c i a l t y markets where dimens i o n a l s t a b i l i z a t i o n i s a l s o a requirement, the chemical cost can be 2-3 times h i g h e r . In summary, the chemicals to be l a b o r a t o r y tested must be capable of r e a c t i n g with wood hydroxyls under n e u t r a l or m i l d l y a l k a l i n e c o n d i t i o n s a t temperatures below 120° C. The chemical system should be simple and capable of s w e l l i n g the wood s t r u c ture to f a c i l i t a t e p e n e t r a t i o n . The complete molecule must react q u i c k l y t o the wood components y i e l d i n g s t a b l e chemical bonds and the t r e a t e d wood must s t i l l possess the d e s i r a b l e p r o p e r t i e s of the wood. One r e a c t i o n system which meets the requirements i s the base c a t a l y z e d r e a c t i o n o f a l k y l e n e oxides with hydroxyl group.
R-CH-CH^HOR
1
HQ"
R-CH-CH^-O-R
1
OH The r e a c t i o n i s f a s t , complete, generates no byproducts, forms s t a b l e ether bonds, and i s c a t a l y z e d by a v o l a t i l e organic amine. A f t e r the i n i t i a l r e a c t i o n , a new hydroxyl group o r i g i n a t i n g from the epoxide i s formed. From t h i s new h y d r o x y l , a polymer begins to form. Due to the i o n i c nature of the r e a c t i o n and the a v a i l a b i l i t y of a l k o x y l ions i n the wood components, the chain length i s probably short due to c h a i n t r a n s f e r . Considering the a l k y l e n e oxides or epoxides i n l i g h t of the preceeding requirements, the lowest member i n the s e r i e s ( e t h y l ene oxide) i s a gas a t room temperature. Ethylene oxide
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120 Reagent
B o i l i n g point °C.
Ethylene oxide Propylene oxide Butylène oxide Epifluorohydrin Epichlorohydrin Epibromohydrin
RESEARCH
10.7 35 63 85 116 135
Trimethylamine Triethylamine
2.9 90
c a t a l y z e d with trimethylamine have been used to react with c e l l u l o s e , but h i g h pressure equipment must be used. Propylene oxide or butylène oxide are l i q u i d s a t room temperature and both can be c a t a l y z e d with t r i e t h y l a m i n e . Of the s u b s t i t u t e d halogen epoxides, e p i f l u o r o h y d r i n i s p r e f e r r e d but i t s cost p r o h i b i t s i t s use ($8/g). The b o i l i n g p o i n t of e p i c h l o r o h y d r i n i s higher but can be e a s i l y removed from the wood a f t e r r e a c t i o n . To determine i f the r e a c t i o n system was capable of s w e l l i n g the wood s t r u c t u r e , the s w e l l i n g c o e f f i c i e n t o f each separate r e agent was determined. A southern pine block (3/4" χ 3/4" χ 4") was immersed i n each separate reagent f o r 1 hour, 150 p s i at 120° C. The b l o c k volume was determined ovendry before treatment and wet immediately a f t e r treatment. The weight g a i n during E f f e c t o f Chemical Reagents on the Swelling of Wood, 120° C, 150 p s i , 1 Hr.
Reagent
Water T r i e thylamine Propylene oxide Epichlorohydrin
Swelling coefficient S 10 .7 5.6 5.8
Weight add-on % 0 .2 3.8 7.6
treatment i s the d i f f e r e n c e between ovendry weight before t r e a t ment and ovendry again a f t e r treatment. I t might be expected that the a l k a l i n e amine would s w e l l the wood as does amines such as p y r i d i n e ; however, t r i e t h y l a m i n e does not s w e l l wood. The s w e l l i n g a b i l i t y of propylene oxide and e p i c h l o r o h y d r i n are about 60% that o f water. So i n the epoxide r e a c t i o n system, i t i s the epoxide that s w e l l s the wood s t r u c t u r e . The amount o f c a t a l y s t needed was determined by r e a c t i n g southern yellow pine with v a r y i n g r a t i o s of epoxide/amine. From t h i s data,
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R a t i o PO/TEA
Reactions
Wt.
20/80 50/50 80/20 90/10 95/5 97/3
121
% Add-On 20 44 52 53 50 40
a r a t i o of 95/5 epoxide to c a t a l y s t was chosen f o r maximum r e a c t i o n w i t h minimum reagent recovery. The c o n d i t i o n s of 120° C. at 150 p s i were chosen f o r a l l runs. By v a r y i n g the r e a c t i o n time, samples were prepared with polymer add-on l e v e l s of from 7 to 60% by weight. Changes i n Volume of Southern Yellow Pine A f t e r Treatment with A l k y l e n e Oxides
Compound
Propylene oxide Butylène oxide Propylene oxide Propylene oxide Epichlorohydrin
Green volume In. 3 3.48 3.60 3.66 3.60 3.60
Ovendry volume before In. 3 3.24 3.24 3.42 3.30 3.36
Ovendry volume after In. 3 3.42 3.60 3.66 3.66 3.72
Weight add-on
% 15.9 21.1 26.1 34.1 41.0
At a weight percent add-on of approximately 20%, the volume of the t r e a t e d wood i s equal to the untreated green volume. Above about 30% weight add-on, the volume of the t r e a t e d wood i s l a r g e r than that of the green wood. These r e s u l t s show that the polymer i s l o c a t e d i n the c e l l w a l l . A d d i t i o n a l evidence o f t h i s i s shown i n the dimensional s t a b i l i t y ( a n t i s h r i n k e f f i c i e n c i e s ) of epoxide-treated wood. A n t i s h r i n k e f f i c i e n c i e s were determined by water soaking t r e a t e d and untreated samples f o r 7 days and measuring the change i n volume due to water a d s o r p t i o n . The h i g h e s t a n t i s h r i n k
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Antishrink
E f f i c i e n c y (R) of Southern Yellow Pine Blocks Weight add-on %
R
Propylene oxide
20.4 28.0 33.8 37.7 51.1
51.3 68.1 66.2 35.4 25.2
Epichlorohydrin
24.5
68.8
Butylène oxide
21.1
68.8
Compound
e f f i c i e n c i e s (R) are i n the range of 21-28% weight add-on. Above t h i s l e v e l , the R values s t a r t to drop o f f which may mean the polymer loadings are so high they have broken the c e l l w a l l and a l l o w the wood to superswell above the green volume. In the e p i c h l o r o h y d r i n samples, the c h l o r i n e was confirmed to be i n the c e l l w a l l by energy d i s p e r s i v e a n a l y s i s of X-ray s p e c t r a generated i n the scanning e l e c t r o n microscope. The greatest percentage of c h l o r i n e was i n the S 2 l a y e r of the c e l l w a l l which i s the t h i c k e s t c e l l w a l l component and contains the most c e l l u l o s e . E l e c t r o n micrographs a l s o showed no polymer i n the lumen, but d i d show changes i n the nature of the c e l l w a l l . These f i n d i n g s of r e t e n t i o n of c h l o r i n e i n the r e a c t i o n of e p i c h l o r o h y d r i n under a l k a l i n e c o n d i t i o n s c o n t r a d i c t somewhat the l i t e r a t u r e on the mechanism of the r e a c t i o n . The r e a c t i o n as
R'0-CH -CH-CH -0R 2
2
OH
4
H0"
CH -CH-CH -OR+HCI 2
2
R'OH
shown i s reported to take place under s t r o n g l y a l k a l i n e condit i o n s (NaOH). The c h l o r o h y d r i n undergoing i n t e r n a l r e a c t i o n to give a new epoxide and HC1 s p l i t out. The new epoxide would then be a v a i l a b l e f o r c r o s s l i n k i n g . This r e a c t i o n does not take place when the r e a c t i o n i s c a t a l y z e d by t r i e t h y l a m i n e . There i s no l o s s of c h l o r i n e during the r e a c t i o n and, i f HCl was formed, there would be a drop i n the pH a f t e r r e a c t i o n . No pH drop was observed.
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The e f f e c t i v e n e s s of these epoxide treatments as decay r e tardants was determined by s o i l - b l o c k t e s t s using two brown-rot f u n g i . The brown r o t t i n g f u n g i are those which p r e f e r e n t i a l l y attack c e l l u l o s e i n the wood l e a v i n g the l i g n i n alone. Treated and untreated southern yellow pine blocks were placed i n t e s t with the fungus Lentinus lepideus and s e p a r a t e l y with L e n z i t e s trabea. Samples were removed at 6 and 12 weeks, and the extent of decay was determined by ovendry weight l o s s . The sample blocks from the r e d u c t i o n i n water s w e l l i n g t e s t (7 days leaching) were a l s o put i n t e s t to determine the changes i n decay r e s i s t a n c e as a f f e c t e d by l e a c h i n g . S o i l - B l o c k Tests on Propylene Oxide Treated Southern Yellow Pine Inoculated with Lentinus lepideus Percent weight add-on
0 5.1 24.0 36.6 44.5 50.9
Percent weight l o s s i n weeks (12) (6) 24.3 8.1 3.2 2.6 3.4 3.7
44.2 17.5 4.8 4.6 7.3 5.3
A weight l o s s a f t e r 12 weeks of l e s s than 5% i s regarded as a positive result. Propylene and butylène oxides and e p i c h l o r o h y d r i n a l l show good decay r e s i s t a n c e to Lentinus lepideus at l e v e l s of about 23% and above. For southern yellow pine, L e n z i t e s trabea i s a much more severe decay fungus.
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S o i l - B l o c k Tests on Treated Southern Yellow Pine Inoculated with Lerizites trabea Nonleached Sample
Leached
Percent weight l o s s a f t e r 6 weeks
12 weeks
6 weeks
12 weeks
Control
44.6
62.9
44.9
68.7
Propylene oxide, 20% 24 37 50
12.9 10.3 8.4 6.5
40.0 35.5 28.7 25.2
26.7 17.3 14.2 12.7
38.6 50.4 23.6 25.0
E p i c h l o r o h y d r i n , 17% 25 35 41
4.9 2.6 2.2
7.2 5.1 5.9
9.7
—
—
6.2 2.4 2.0 3.7
Butylène oxide,
5.2 2.9 3.2
7.0 1.8 2.7
18.8 11.9 2.0
7% 14 23
18.8 12.4 3.8
—
4.1 4.0
and the propylene oxide treatment does not hold up. For trabea, butylène oxide and e p i c h l o r o h y d r i n g i v e good decay r e s i s t a n c e a t l e v e l s above 22%. In c o n c l u s i o n , the data from t h i s work show that propylene oxide, butylène oxide, and e p i c h l o r o h y d r i n treatments g i v e good dimensional s t a b i l i t y to water s w e l l i n g at precent weight add-ons of approximately 25%. At these same l e v e l s of chemical s u b s t i t u t i o n , two of the treatments show good r o t r e s i s t a n c e . These treatments may f i n d a p p l i c a t i o n s i n products such as window u n i t s and millwork i n which r e s i s t a n c e to s w e l l i n g from water i s as important as r o t r e s i s t a n c e . Tests are now i n progress to determine s t r e n g t h l o s s , i f any, i n the treated wood as w e l l as s t u d i e s on weathering, g l u i n g , p a i n t a b i l i t y , and burning c h a r a c t e r i s t i c s .
