JPH0482703A - Production of modified wood - Google Patents

Abstract

PURPOSE:To make a wood have high dimensional stability throughout, reduce the degree of deterioration in strength and maintain a wood feeling on the surface of the wood, by impregnating a raw material wood with an acrylamide and a polymerization initiator, and treating the wood at a specified temperature. CONSTITUTION:An acrylamide and a polymerization initiator are infiltrated into a raw material wood in the state of being dissolved in water. The wood thus impregnated is then treated by one of the following methods (1) to (3), whereby an insoluble hardened resin is formed and fixed in the texture of the- wood. (1) Once heating in a comparatively low temperature range of about 60-90 deg.C, and then heating in a high temperature range of about 100-150 deg.C. (2) Heating only in the comparatively low-temperature range of about 60-90 deg.C. (3) Heating only in the high temperature range of about 100-150 deg.C. The polymerization initiator may be, for example, a radical polymerization initiator such as hydrogen peroxide, potassium persulfate and ammonium persulfate. The heating may be carried out by a hot-air drying method, a high-frequency or electron ray heating method or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for producing modified wood used for housing equipment, building materials, etc.

[Conventional technology]

Conventionally, methods for imparting dimensional stability to wood include, for example, the following methods (1) to (2).

■ After impregnating the wood with acetic anhydride and a catalyst such as potassium acetate, by heating it at 100 to 130°C,
A method of replacing the hydroxyl groups of wood cellulose with acetyl groups (acetylation). According to this method, the hydrophilic hydroxyl groups of wood cellulose are replaced with hydrophobic acetyl groups, so
As a result of the reduced hygroscopic (water) property of the wood, dimensional changes in the wood are reduced.

(2) A method in which resin is produced in wood by impregnating it with styrene, methyl methacrylate, etc., and then heating or irradiating it with electron beams (WPC manufacturing method). According to this method, in addition to improving dimensional stability, hardness and wear resistance are also improved.

■ A method of imparting high dimensional stability by impregnating wood with a resin having an isocyanate group or an epoxy group and reacting the resin with the wood [Problem to be solved by the invention] However, methods of The methods are as follows (0')
I had ~(one) problem.

(7) In the method (2) above, it is difficult to completely acetylate the cellulose inside the wood, and therefore the inside of the wood cannot be dimensionally stabilized. In addition, a sufficient acetylation rate cannot be obtained and unreacted acetic acid remains in the wood.
You will smell acetic acid, and acetic acid will destroy the wood structure and reduce the strength of the wood.

(a) In the method (2) above, since the monomer impregnated into the wood is hydrophobic, the same monomer is not impregnated into the cell walls of the wood. Therefore, in order to impart high dimensional stability, it is necessary to impregnate the wood in a large amount of 50 to 100% by weight based on the weight of the wood before impregnation. the result,
The wood texture of the wood surface disappears.

(1) In method (2) above, there is a possibility that the resin that is impregnated into the wood and reacted with the wood components will harden, and if the resin hardens, the degree of curing shrinkage will be large and the wood structure will be destroyed. As a result, the wood swells more than untreated wood, reducing dimensional stability and reducing the strength of the wood.

In view of these circumstances, the present invention has been developed to efficiently produce modified wood that is dimensionally stable to the inside of the wood, has less residual unreacted chemicals, has less strength deterioration, and does not lose its surface wood texture. The objective is to provide a method that can obtain a good result.

[Means to solve the problem]

In order to solve the above problems, the method for producing modified wood according to the present invention involves impregnating raw material wood with acrylamides and a polymerization initiator in a state dissolved in water, and then impregnating the raw material wood with the following (al, (By performing one method selected from bl and <cr, an insoluble cured resin is produced and fixed within the wood structure.

fa) Once heated within a relatively low temperature range of about 60 to 90°C, then heated within a high temperature range of about 100 to 150°C.

(b) Heating only within a relatively low temperature range of about 60-90°C.

(c) Heating only within a high temperature range of about 100-150°C.

The raw material wood to be modified in the present invention is not particularly limited, and includes, for example, raw wood logs, sawn timber products, sliced veneers, rotary veneers, plywood, and the like. There are no limitations on the tree species, etc.

The acrylamide used in this invention is not particularly limited, but includes, for example, acrylamide, methylol acrylamide, dimethylol acrylamide, methacrylamide, methylol methacrylamide, dimethylol methacrylamide, and the like. These are 1
Only the seeds may be impregnated into the raw material wood and homopolymerized within the wood, or multiple species may be impregnated into the raw material wood and they may be copolymerized within the wood. When using a mixture of multiple types, the mixing ratio for the acrylamide intercondylar floor is as follows:
Not particularly limited.

Since these acrylamides have hydrophilic groups such as amide groups and methylol groups, they are soluble in water, and by impregnating them in a dissolved state in water, it is possible to impregnate the inside of wood cell walls. Acrylamides also have vinyl groups and can be easily polymerized by heating, etc., so after impregnating them into wood, the wood is heated and the acrylamides undergo a polymerization reaction to form a polymer. By forming an acrylamide resin, high dimensional stability can be imparted to the interior of the wood. but,
When the acrylamide resin is polymerized by low-temperature heating, it may be water-soluble, so there is a risk that it will seep out of the wood due to water or moisture, reducing the dimensional stability of the wood. Therefore, as will be explained in detail later, the acrylamide resin is heated at a relatively low temperature and/or high temperature to crosslink (cure) the acrylamide resin, thereby ensuring that it becomes insoluble in water. The crosslinking reaction of acrylamide resin is
Through functional groups such as vinyl groups and amide groups in the same resin,
This occurs between and within the molecules of the resin. In particular, when the acrylamide resin has a methylol group, the methylol groups within and between molecules of the resin are also crosslinked by condensation, resulting in an insoluble cured resin with a higher degree of crosslinking. If the acrylamide resin produced within the wood becomes insoluble in water, there is no risk that the resin will seep out of the wood due to water or moisture, and it will be fixed within the wood, resulting in modified wood with high dimensional stability. At the same time, the water resistance and moisture resistance of the same wood are also improved. Furthermore, as will be explained in detail later, the acrylamide resin can undergo a condensation reaction (crosslinking reaction) with hydrophilic hydroxyl groups in cellulose, which is a wood component, to make the hydroxyl groups hydrophobic when heated at high temperatures. It is possible. If the hydroxyl group becomes hydrophobic, the water absorption and hygroscopicity of the wood will decrease, so the dimensional stability of the wood will further improve. Further, if necessary, the raw material wood may be impregnated with a water-soluble monomer other than acrylamide, and the acrylamide and the above-mentioned hetamine may be copolymerized within the wood. This is because, depending on the type of monomer, it is possible to impart flame retardancy, abrasion resistance, etc. to wood, or to improve the hardness of wood.

In this invention, in order to efficiently polymerize and harden (crosslink) the acrylamides in wood, in addition to the acrylamides, a polymerization initiator needs to be impregnated into the raw material wood in a state dissolved in water. The agent is not particularly limited as long as it is water-soluble, and examples thereof include radical polymerization initiators such as hydrogen peroxide, potassium persulfate, and ammonium persulfate. These radical polymerization initiators can also be used as a redox catalyst initiator in combination with a reducing agent. The polymerization initiator may be used alone or in combination.

Further, if necessary, the acid catalyst may also be impregnated into the raw material wood in a state of being dissolved in water.

For example, when the acrylamide has a methylol group such as methylolacrylamide, it is impregnated for the purpose of promoting the condensation of the methylol group as described above. Such acid catalysts are not particularly limited as long as they are water-soluble, but examples include inorganic acids such as hydrochloric acid and phosphoric acid, their salts ammonium chloride, ammonium hydrogen phosphate, and paratoluene. Examples include organic acids such as sulfonic acid. Among these acid catalysts, those that are substantially neutral at room temperature but become acidic by dissociating the acid when heated, such as ammonium chloride and ammonium hydrogen phosphate, are preferred.

When used, acid catalysts may be used alone or in combination.

As a method of impregnating raw wood with acrylamides and a polymerization initiator (hereinafter collectively referred to as "impregnation components") dissolved in water (hereinafter referred to as "impregnation treatment liquid"), Examples include, but are not limited to, a reduced pressure impregnation method, a pressure impregnation method, a normal pressure dipping method, a coating method, and the like. Before impregnating with the impregnating treatment liquid, it is recommended that the raw material wood be subjected to a water saturation treatment in advance. This is because the impregnating components contained in the impregnating solution are quickly impregnated (diffused) into the wood using the water in the wood as a medium, so the impregnation time can be shortened. The method of water saturation treatment is not particularly limited, but may be carried out by underwater storage, steaming, reduced pressure impregnation, pressurized impregnation, or the like. Note that when impregnating with the impregnating treatment liquid under reduced pressure, it is not necessarily necessary to perform this water saturation treatment.

In addition, in order to improve processing efficiency, impregnation treatment with impregnating components is usually carried out by including all the necessary impregnation components in one impregnation treatment solution and impregnating the wood with the same impregnation treatment solution. However, it is not limited to this. For example, each impregnating component may be separately impregnated with a plurality of types of impregnating treatment liquids. That is, the impregnation timing of each impregnating component is not limited at all as long as it is before the heat treatment described below.

The concentration of acrylamide in the impregnating solution is not particularly limited, but is preferably about 10 to 50% by weight.

The following (al, (b) and (
By performing one method selected from c), an insoluble cured resin is generated and fixed within the wood structure.

(a) Once heated within a relatively low temperature range of about 60 to 90°C, then heated within a high temperature range of about 100 to 150°C.

(b) Heating only within a relatively low temperature range of about 60-90°C.

(c) Heating only within a high temperature range of about 100-150°C.

The heating method is not particularly limited, but includes, for example, a method of drying the impregnated wood with hot air, a method of heating with high frequency waves or an electron beam, and the like.

Polymerization and curing (crosslinking) of acrylamides is 60 to 90
This can be done by heating at relatively low temperatures of the order of °C. In particular, it is preferable to heat at a temperature of about 60 to 70°C. This is because when polymerized and cured at such a temperature, the degree of curing shrinkage of the acrylamide resin becomes smaller, and the bulking effect becomes higher, thereby further improving the dimensional stability of the wood.

Furthermore, in order to further improve the dimensional stability of wood, as described above, it is effective to cause the hydrophilic hydroxyl groups of wood cellulose to undergo a crosslinking reaction with acrylamides to make the hydroxyl groups hydrophobic. The hydrophobization reaction of the hydroxyl group is performed at 10
By heating at a high temperature of about 0 to 150℃,
It can be carried out efficiently. Of course, heating at such temperatures also progresses polymerization and curing of acrylamides.

Since the hydrophobization reaction is a condensation reaction, if a large amount of water is present in the wood, the reaction rate tends to be slow due to chemical equilibrium. Therefore, in order to allow the hydrophobization reaction to proceed as efficiently as possible, it is desirable to remove as much moisture as possible from the pre-stamped wood. In order to carry out such a process, although not particularly limited, the wood after impregnation treatment is first treated with a 60%
After heating at a relatively low temperature of about ~90°C to advance the polymerization and curing reaction of acrylamide and lowering the moisture content of the wood, heating at a high temperature of about 100 to 150°C produces acrylamide resin. It is desirable to allow the crosslinking reaction between the clay floor and the acrylamide resin and wood cellulose to proceed. In this way, heating is performed sequentially within two temperature ranges (method of fa) above)
By doing so, the above-mentioned bulking effect and the effect of making the hydroxyl groups of wood cellulose hydrophobic can be obtained, so that higher dimensional stability can be obtained.

Note that the heat treatment of the wood after the impregnation treatment may be carried out by sequentially heating within two temperature ranges as in the method (a) above, or in one heat treatment as in the methods (b) and (c1) above. Heating may be performed only within one temperature range.The heating time within each temperature range is not particularly limited.

In the above manner, insoluble hardened resin is generated within the wood.
After fixing, the surface of the wood is washed with water, if necessary, and dried to obtain the desired modified wood.

[For production]

If raw wood is impregnated with a component that produces an insoluble cured resin that has a dimensional stabilizing effect, dissolved in water, and then allowed to react within the wood, the aqueous solution will have a low permeability into the wood. Because of this, after the component is impregnated into the wood structure, it changes into the insoluble cured resin. As a result, the interior of the wood is highly dimensionally stabilized. Since the components react at a high reaction rate, the amount of unreacted drug remaining is reduced. Since the above-mentioned components have a small degree of curing shrinkage, the degree of destruction of the wood structure is reduced, so that deterioration in the strength of the wood is suppressed. As mentioned above, the above ingredients are efficiently impregnated into the inside of the wood, so the amount of impregnation required for dimensional stabilization is small.Furthermore, after impregnation, the ingredients become insoluble in water and are fixed inside the wood, leaving them on the wood surface. Since there is no risk of it seeping out, the wood texture of the wood surface is maintained.

〔Example〕

Hereinafter, specific examples of the present invention will be described in detail together with comparative examples, but the present invention is not limited to the following examples.

Example 1 Water: methylol acrylamide (hereinafter referred to as rMAM)
(abbreviated as J) - 0.5% by weight of potassium persulfate based on MAM as a radical polymerization initiator and M as an acid catalyst in an aqueous solution prepared by mixing at a ratio of 4:1 (weight ratio).
3% by weight of ammonium chloride based on AM was added and mixed. Five agathis veneers each having a width and length of 220 nm and a thickness of 3 cranes were impregnated with this aqueous solution under reduced pressure (50 wHg), and then impregnated for one day by a normal pressure immersion method.

Thereafter, the veneer was heated at 70° C. for 6 hours and then at 150° C. for 2 hours to obtain a modified wood veneer in which an insoluble cured resin was generated and fixed inside.

The impregnation rate expressed by the following formula (1) was investigated for the obtained modified wood veneer, and the results are shown in Table 1. A test piece of the end surface of the wood was treated in the same way as the veneer above,
After obtaining the end face test piece of the modified wood, the dimensional stability of the modified wood was examined. Dimensional stability is expressed by the following formula (2)
Evaluated by the anti-hygroscopic ability expressed by the following formula (3) and the anti-water absorption capacity expressed by the following formula (4),
The results are shown in Table 1.

(In formula (2), B1 represents the dry size of untreated wood, and B2
B represents the untreated wood saturated size and B represents the treated wood dry size. ) (In formula (3), S represents the hygroscopic swelling rate of untreated wood,
St represents the hygroscopic swelling rate of the treated wood. ) (in formula (4),
T represents the water absorption swelling rate of untreated wood, and T represents the water absorption swelling rate of treated wood. ) Example 2 A radical polymerization initiator was added to an aqueous solution prepared by mixing water:MAM=2:1 (ffi ratio).
Potassium persulfate (0.5% by weight based on MAM) and ammonium chloride (3% by weight based on MAM) as an acid catalyst were added and mixed. This aqueous solution was applied under reduced pressure to 5 Agatis veneers each measuring 220 mm in width and length and 3 mm in thickness.
50++nHg) and then impregnated for one day by normal pressure immersion method.

Thereafter, the veneer was heated at 70° C. for 6 hours and then at 150° C. for 2 hours to obtain a modified wood veneer in which an insoluble cured resin was generated and fixed inside.

The impregnation rate of the obtained modified wood veneer was examined in the same manner as in Example 1, and the results are shown in Table 1.

In addition, a test piece of the end surface of agathis wood with a width and length of 40 m and a thickness of 5 fl was subjected to the same treatment as the veneer described above to obtain a test piece of the modified wood end surface. The dimensional stability of the modified wood was examined in the same manner as in Example 1,
The results are shown in Table 1.

Example 3 - MAM was added as a radical polymerization initiator to an aqueous solution prepared by mixing water:MAM at a ratio of 4:1 (weight ratio).
Potassium persulfate (0.5% by weight based on MAM) and ammonium chloride (3% by weight based on MAM) as an acid catalyst were added and mixed. The width and length of this aqueous solution are 2
At 2ON, reduce the pressure (50
gmHg) and then impregnated for one day by normal pressure immersion method.

Thereafter, the veneer was heated at 70° C. for 24 hours to obtain a modified wood veneer in which an insoluble cured resin was generated and fixed.

The impregnation rate of the obtained modified wood veneer was examined in the same manner as in Example 1, and the results are shown in Table 1.

In addition, a test piece of the end surface of agathis wood having a width and length of 40 m and a thickness of 5 tl was treated in the same manner as the veneer described above to obtain a test piece of the modified wood end surface. The dimensional stability of the modified wood was examined in the same manner as in Example 1,
The results are shown in Table 1.

Example 4 MAM was added as a radical polymerization initiator to an aqueous solution prepared by mixing water:MAM at a ratio of 4:1 (weight ratio).
Potassium persulfate (0.5% by weight based on MAM) and ammonium chloride (3% by weight based on MAM) as an acid catalyst were added and mixed. The width and length of this aqueous solution are 2
At a distance of 20 m, 5 Agatis veneers with a thickness of 3 cranes were decompressed (50
mHg) and then impregnated for one day by normal pressure immersion method.

Thereafter, the veneer was heated at 110° C. for 24 hours to obtain a modified wood veneer in which an insoluble cured resin was generated and fixed.

The impregnation rate of the obtained modified wood veneer was examined in the same manner as in Example 1, and the results are shown in Table 1.

In addition, a test piece of the end surface of agathis wood having a width and length of 40 m and a thickness of 51 m was treated in the same manner as the veneer described above to obtain a test piece of the modified wood end surface. The dimensional stability of the modified wood was examined in the same manner as in Example 1,
The results are shown in Table 1.

Example 5 - MAM was added as a radical polymerization initiator to an aqueous solution prepared by mixing water and MAM at a ratio of 4:1 (weight ratio).
Potassium persulfate (0.5% by weight based on MAM) and ammonium chloride (3% by weight based on MAM) as an acid catalyst were added and mixed. The width and length of this aqueous solution are 2
Decompress (50
4Hg) After impregnating, pressure impregnation was carried out for 1 hour under a pressure of 10 kg/aa, and further impregnation was carried out for one day by normal pressure immersion method.

Thereafter, the veneer was heated at 70° C. for 6 hours and then at 150° C. for 2 hours to obtain a modified wood veneer in which an insoluble cured resin was generated and fixed inside.

The impregnation rate of the obtained modified wood veneer was examined in the same manner as in Example 1, and the results are shown in Table 1.

In addition, a butt side test piece of agathis wood with a width and length of 40 mm and a thickness of 50 mm was subjected to the same treatment as the veneer described above to obtain a butt side test piece of modified wood. The dimensional stability of the modified wood was examined in the same manner as in Example 1, and the results are shown in Table 1.

Comparative example - water: polyethylene glycol monomethacrylate) (P
EGMA)=2:1 (weight ratio) to an aqueous solution prepared by adding PE as a radical polymerization initiator.
Potassium persulfate (1% by weight based on GMA) was added and mixed. This aqueous solution was applied under reduced pressure (50 mHg) to 5 Agatis veneers with a width and length of 220 days and a thickness of 3 cranes.
After impregnating, it was impregnated for one day by the normal pressure immersion method.

Thereafter, the veneer was heated at 70° C. for 24 hours to obtain a modified wood veneer in which resin was generated inside.

The impregnation rate of the obtained modified wood veneer was examined in the same manner as in Example 1, and the results are shown in Table 1.

In addition, a test piece of the end surface of agathis wood with a width and length of 401 m and a thickness of 5 tl was treated in the same manner as the veneer described above to obtain a test piece of the modified wood end surface. The dimensional stability of the modified wood was examined in the same manner as in Example 1, and the results are shown in Table 1.

Table 1 As shown in Table 1, the modified wood obtained in Examples 1 to 5 has almost the same bulking properties as the modified wood obtained in Comparative Example, but It can be seen that the anti-water absorption ability is high (and the dimensional stability is extremely excellent).

〔Effect of the invention〕

According to the method for producing modified wood according to the present invention, the interior of the wood is highly dimensionally stabilized, and there is little residual odor from unreacted chemicals.
It is possible to efficiently obtain modified wood with a small degree of deterioration in strength and with no loss of surface wood texture.

In addition, since the active ingredients generated inside the modified wood are insoluble in water, there is no risk of them seeping out onto the wood surface due to water or moisture, so the modified wood has good water resistance and moisture resistance. Are better.

Agent: Patent Attorney Takehiko Matsumoto Procedural Amendment (Spontaneous May 2, 1991, title of invention: Process for producing modified wood 3, relationship with the case of the person making the amendment Patent applicant address: 1048 Kadoma, Kadoma City, Osaka Prefecture) Address name (583) Matsushita Electric Works Co., Ltd. 4, No agent 7 days 6, ?i! i Positive subject specification 7, ?! Positive contents ■ The full text of the scope of claims column of the specification is as follows: correct.

- After impregnating the raw material wood, the following (
A method for producing modified wood in which an insoluble cured resin is produced and fixed in the wood structure by performing one method selected from al, (b) and (c1).

(al) Once heated within a relatively low temperature range of about 60 to 90°C, then heated within a high temperature range of about 100 to 150°C.

(b) Heating only within a relatively low temperature range of about 60-90°C.

(It is heated only within a high temperature range of about 1100 to 150 degrees Celsius.) ■ Page 3 of the specification, lines 6 to 10, 1171 In the method (■) above, unreacted acetic acid remains in the wood.''
``C7) In the method ① above, acetic acid, which is a by-product, remains in the wood, so correct it to ``J''.

■ On page 4, lines 1 and 2 of the specification, the phrase ``The reacted resin may harden, and if the resin hardens, its curing shrinkage'' is replaced by ``the reacted resin itself may harden.''"ofcontraction," he corrected.

■ On page 4, lines 6-7 of the specification, it says, ``This invention achieves dimensional stability even inside the wood, and there is little residual unreacted chemical.''"Not a lot," he corrected.

■ Page 4, lines 12 to 14 of the specification states, ``The method for producing modified wood according to the present invention uses acrylamides and a polymerization initiator,
The phrase "dissolved in water" should be corrected to read, "The method for producing modified wood according to the present invention involves dissolving acrylamides having methylol groups, a polymerization initiator, and an acid catalyst in water." .

■ On page 5, lines 11 to 15 of the specification, the phrase "acrylamides used in this invention...methacrylamide, methylolmethacrylamide" was replaced with "acrylamides having a methylol group used in this invention (hereinafter referred to as Unless otherwise specified, "acrylamides" refers to "acrylamides having a methylol group.") is not particularly limited, but is corrected to, for example, methylol acrylamide, dimethylol acrylamide, methylol methacrylamide.

■ Insert the following sentence between the first and second lines of page 6 of the specification.

``If necessary, in addition to acrylamides having a methylol group, at least one type of acrylamide without a methylol group such as acrylamide or methacrylamide may be dissolved in water and impregnated into the wood. , they may be copolymerized within the wood.'' ■ Page 6 of the specification, lines 10 to 11, ``By forming a resin, high dimensional stability extends to the inside of the wood.''"By forming a resin, the wood has high dimensional stability."

■ From the second line from the bottom of page 6 to the sixth line of page 7 of the specification, ``Crosslinking reaction of acrylamide resin... is produced.''

Acrylic acid formed in wood progresses by the condensation of methylol groups in acrylamide.

"Acrylic formed within wood" is corrected.

[Phase] In the third line from the bottom of page 8 of the specification to the third line of page 9, “
In addition, if necessary, it is impregnated for purposes such as acid catalyst. ``In this invention, in addition to the acrylamides and the polymerization initiator, it is also necessary to impregnate the raw material wood with an acid catalyst dissolved in water. This is to promote the formation of methylol groups and the condensation between the methylol groups and the hydrophilic hydroxyl groups of the wood component.This is corrected as J.

(2) On page 9, lines 12-13 of the specification, the phrase ``When used, the acid catalyst, alone or alternatively'' is corrected to ``The acid catalyst is used alone or alternatively.''

@ On page 9, line 15 of the specification, ``acrylamides and polymerization initiators (hereinafter referred to as these)'' has been corrected to ``acrylamides, polymerization initiators, and acid catalysts (hereinafter referred to as these'').

0 On page 14 of the specification, lines 11 to 13, it says, "It changes. As a result, there is less unreacted chemical remaining inside the wood." The above components convert into a resin with almost no odor at a high reaction rate, and there are almost no odor-bearing by-products, so the modified wood has less odor.''

■ On page 24, lines 7 to 9 of the specification, the statement ``According to the manufacturing method of modified wood... there are few remaining tools and the strength is deteriorated'' has been changed to ``
"According to the manufacturing method of modified wood, it is highly dimensionally stable, has little odor, and has reduced strength."

Claims (1)

[Claims] 1. After impregnating raw material wood with acrylamides and a polymerization initiator dissolved in water, one of the following (a), (b), and (c) is applied to the raw material wood. A method for producing modified wood that generates and fixes an insoluble cured resin within the wood structure by performing one selected method. (a) Once heated within a relatively low temperature range of about 60 to 90°C, then heated within a high temperature range of about 100 to 150°C. (b) Heating only within a relatively low temperature range of about 60-90°C. (c) Heating only within a high temperature range of about 100-150°C.