Temperature Needed to Bend Wood

[FredFord]

I would like to construct a table or graph that shows the glass transition temperature (the temperature at which wood can be bent because the lignin has been softened) for lignin (the part of wood that holds the fivers together the glue in wood) vs. the percent moisture content of the wood.

I know that dry lignin (zero moisture) requires a temperature of 338 degrees F. With steam, approximately 200 degrees F in the wood, I can bend wood with as low as 10% moisture content. I can also bend green wood at ambient temperature and it will only set if I secure it until it dries. These observations are antidotal except for your 388 degrees F.

If you have any information about how the lignin in wood behaves with temperature and moisture please share it with us.

I believe that if we could publish the table or graph I want to construct it would put a lot of things into perspective for woodworkers that want to bend wood.

 

[Trent Mason]

I am by no means an expert on this, actually more like an idiot. It is very interesting none the less. Wouldn't the temperature/moisture content be different for different species of wood?

 

[CarvedTones]

My experience is obviously anecdotal, but it varies considerably depending on thickness and technique/equipment (using a bending strap for example). Taking thickness (or thinness) to an extreme, boats are cold molded with wood veneers. I bent the maple sides on this at room temperature:

 

[FredFord]

I am by no means an expert on this, actually more like an idiot. It is very interesting none the less. Wouldn't the temperature/moisture content be different for different species of wood?

My questions is given a specific moisture content then what is the temperature at which the lignin becomes soft.

Moisture content varies first by the extent the wood has been dried. It starts off at 100% or so. Once dried then the major factor in moisture content is the relative humidity of the air it is stored in. Then the method of drying comes to play. Air dried wood will have higher moisture content than kiln dried for the same relative humidity. Last is the variation between species and even variances between trees of the same species.

I believe the lignin is a common polymer found in all woods and should become soft at a given temperature and moisture point. Becoming soft is only the major factor in bending but not the only one and most species will be good at bending or not because of many other factors

 

[FredFord]

My experience is obviously anecdotal, but it varies considerably depending on thickness and technique/equipment (using a bending strap for example). Taking thickness (or thinness) to an extreme, boats are cold molded with wood veneers. I bent the maple sides on this at room temperature:

You are looking at cold bending. Laminate bending relies on glue to hold the thin strips in place and the lignin is never heated. If you did not heat your wood then you are using the same process and a mechanical anchor must be used to hold the wood in place. Unless of course you were working with green wood. There are many variables. Too many!

In heat bending, most of use steam, the point at which the wood becomes bendable, is a function of moisture and temperature. The thickness has a large effect on the time it takes to heat the center and the mechanical means to bend the wood even when softens.

I am only asking when the wood becomes soft.

 

[Joe Scharle]

It is well known that lignin and the hemicelluloses are amorphous polymers. Cellulose,
on the other hand, is mainly crystalline but does contain regions with amorphous character within the crystalline lattice. Amorphous polymers undergo changes in physical characteristics upon heating resulting in transitions to a glassy and rubbery state (Irvine 1985). The temperature where this happens is referred to as the glass transition temperature or Tg. Transitions from one state to another depend on sufficient energy input to overcome the intermolecular attractive forces with in the polymer and allow a greater degree of flow. Chawla and Sharma showed that once the temperature exceeded 160 C°, there was sufficient flow of lignin to block pores within the wood structure to restrict the reentry of moisture (1972). Hillis and Roza found that a minimum heating of 2 hours at 100 C° plasticized the hemicellulose-lignin matrix.
It is also well recognized that moisture lowers the Tg of an amorphous polymer (Goring
1963). Goring examined the softening temperatures for dry and wet lignin, hemicellulose and cellulose and found that the sorption of water greatly decreased the Tg of both lignin and hemicellulose while the Tg for cellulose only varied slightly from dry to wet. The Tg for dry lignin is approximately 155-160 C° depending on the chemical structure. High syringyl content lignins have a lower Tg as compared to other lignins (Olsson and Salmen 1993). Inoue and coworkers showed that at temperatures between 180 and 200 C°, the lignin-hemicellulose matrix became plasticized and underwent thermoplastic flow (Inoue et al. 1993). All of this data shows that under heat and pressure, the lignin-hemicellulose matrix becomes thermoplastic and will deform to a different stable matrix. If the compressed matrix is cooled under pressure, the newly formed matrix has little memory of its precompressed form. Chawla and Sharma published results suggesting that during the heating process crosslinking of polysaccharide chains could occur (1972). They also suggested that some of the thermal degradation products recombined during heating. Norimoto (1994) and Dwianto et al. also suggested the formation of interlinkages between wood polymers during the heat treatment of wood. Boonstra et al. suggested that degradation products from the hemicelluloses and lignin for reactive intermediates that cross link during the curing stage Finally, it has been shown that heating wood under pressure resulted in an increase in cellulose crystallinity (Inoue et al. 1991). This could also have an influence on rewetting of the cell wall polymers after compression.
Based on the literature review, there are many possible chemical reactions taking place during steaming of wood fiber. Of interest to our research program is the loss of hemicelluloses (to decrease hygroscopicity and increase resistance to attack by micro-organisms) and to convert the released free sugars to furan intermediates (that can be converted to furan resins to increase internal bond strength) and to increase in dimensional stability of fiberboards. The effects of the treatments depend on many variables such as temperature, time of treatment, moisture content, chemical composition of the lignocellulosic, the addition of specific catalysts, and probably other factors which are not yet understood.​

see:
http://www.fpl.fs.fed.us/documnts/pdf2002/rowel02a.pdf​

 

[Gofor]

"Based on the literature review, there are many possible chemical reactions taking place during steaming of wood fiber.":rolf::rolf:

Guess that kinda says it.

Go

 

[FredFord]

Joe, thank you very much for this information. I have printed Rogers report and will take some time to understand. I have contacted Roger on this subject but he offered little information. Altough this report deals with wood fiber under pressure it is clear there is an understanding that should be tranlatable to our woodworking domain. I would like some numerical guidelines for us simple folkes to use to bend wood.

I will be in contact when I have digested this report. I may be a while.

Thanks again

 

[Joe Scharle]

Good luck, Fred.

 

[ErnieM]

Huh!??? :dontknow: :dontknow: :dontknow:

Ernie

 

[CarvedTones]

As X approaches zero and light bends like hot taffy...

Even with the thin wood I am used to bending, there is a point at which you compress the fibers on the inside and stretch the fibers on the outside too much and get a tear if you go too thick on too tight of a turn even if you have the wood very hot. The one in the pic above I bent cold, but most of the time I used an aluminum bending strap, an inside form and an iron. I used this technique Fischer, 1974

http://www.scavm.com/awfribs.htm

which recommends 350 F (176.6 C), right in the midle of the numbers in Joe's impressive research.

 

[ScottM]

Fred, I doubt you need this info for the Toys 4 Tots. What project do you have up your sleeve?:dontknow:

 

[FredFord]

No project. I am reviewing a section of a friends book he is writing and this will help me understand what is happening in wood during the bending process and also what moisture content is required.

 

[Joe Scharle]

Fred you might find some help from Richard Jones on knots.

@ http://forums.taunton.com/fw-knots/messages/?msg=44505.1

The link doesn't work like most because of Taunton's old board. However, once in, search on the msg no.