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Файл:Plastics technology. Часть 2. Учебное пособие.pdf
71
conveniently with formic acid, and reacted for a further 5-20 minutes. The
4NH
4
Cl + 6CH
2
O
N
4
(CH
2
)6
+
6H
2
O + 4HCl
resulting resin is then stabilized by neutralizing to a pH 7.5 with alkali to
give a water-soluble resin with an approximately 50% solids content.
When the resin is to be used in aqueous solution, as is normally the case, it
is then partially dehydrated to give a 70% solids content by vacuum
distillation. For some uses, for example for application in tropical
countries, the resin is spray dried to ensure gre ater stab i lity .
The finished product is checked for viscosity, solids content, pH
value (which must be in the range of 7.3-7.5) and of its reactivity with a
standard hardener.
Resins are commonly available with U-F molar ratios ranging from
1:1.05 to 1:2.2. Since formaldehyde is more expensive than urea the high
F-U ratio resins are more expensive. They do, however, have greater
clarity, the best water resistance, marginally superior mechanical
properties, longer shelf life (up to two years) and greatest reactivity. The
degree of condensation is quite important since, if it was insufficient, the
resin would be absorbed into the wood and would thus be unavailable to
act as adhesive. Wood penetration by the adhesive may be reduced by
incorporation into the latter of some rye or wood flour which at the same
time improves adhesion spreading properties and reduces cost.
The resins are hardened by acidic conditions. Phosphoric acid, or
more commonly ammonium chloride, an acid donor, is employed. The
ammonium chloride functions by reaction with formaldehyde to give
hydrochloric acid. Hexamine is also formed during this reaction.
About 1.5 parts ammonium chloride per 100 parts of the resin
solution are generally used. The hardener is added as an aqueous solution.
The resins continue to be used in large quantities in general wood
assembly work. In most cases the resin-hardener mixture is applied to the
surfaces to be joined and then clamped under pressure while hardening
occurs. It is also possible to coat the resin on to one surface and the
hardener on the other surface, allowing them to come into contact in situ
and thus eliminating pot-life problems. Gap-filling resins may be produced
by incorporating plasticizers such as furfuryl alcohol and fillers to reduce
shrinkage and consequent cracking and crazing.
One of the largest applications of U-F resins at the present time is
in the manufacture of chipboard. Wood chips are mixed with about 10% of
resin-hardener solution and the mixture pressed in a multi-daylight press
72
for about eight minutes at 150°C. Since the odour of formaldehyde is
disagreeable it is important that little of the pungent chemical be released
into the press shop during the opening of the presses. For this reason the
resin should have a low free formaldehyde content. Since a low degree of
condensation is desirable to ensure good dispersion, a rather low F-U ratio
is necessary in order to achieve a low free formaldehyde content.
Wood chipboard is free from grain and is thus essentially isotropic
in its behaviour. The mechanical properties are approximately the same as
the average of the properties of the original wood measured along and
across the grain. The water resistance of chipboard is poor but, being
isotropic, it does not warp as long as it is able to swell freely in all
directions.
Foams and firelighters
Foams may be made from urea-formaldehyde resins using simple
techniques. In one process the resin is mixed with a foaming detergent,
whipped up with air in a mixing device and blended with an acid, such as
phosphoric acid, as it leaves the mixer. The foams may be formed in situ in
building cavity walls but, because of the large amounts of water present, it
is necessary that the foam be formed between porous surfaces. Typical
products have a closed cell content of about 80% and have little mechanical
strength, as they are very friable. They have a very low thermal
conductivity with а К value of 0.15-0.20. Foams ranging from 0.008-0.048
g/cm3 may be produced; those with a density of about 0.012g/cm3 having
the lowest conductivity. Such foams are very cheap and are now being
made in situ in building applications in Britain. Foams have also been used
as an aid to floral decoration and in ground form as an artificial snow in
cinema and television productions.
A rather strange but nevertheless large-scale application of U-F
resins is in the manufacture of firelighters, made by a modification of the
foam process. The resin solution is blended with a small amount of
detergent and then whisked with paraffin. A hardener is added and the
resin allowed to set. In effect the product is a U-F foam saturated with
paraffin.
Other applications
Modification of urea-formaldehyde resins with other reagents gives
rise to a number of useful materials. For example, co-condensation of ureaformaldehyde and a monohydric alcohol in the presence of small quantities
73
of an acidic catalyst will involve simultaneous etherification and
+
C
NHCH
2
OH
NHCH2OH
O
HOC4H
9
C
NHCH
2
OC
4H9
NHCH2OH
O
+
H
2
O
resinification. n-propanol, n-butanol and isobutanol are commonly used for
this purpose. As an example n-butanol will react with the methylol urea as
shown in Figure 25.
Figure 25
By varying reaction conditions and reactant proportions differing
products may be obtained. Many of the alkoxy groups are retained during
cure and the resins have a degree of thermoplasticity. Soluble in organic
solvents and used in conjunction with plasticizing alkyd resins, these
materials form useful stoving lacquers. Air-drying lacquers, suitable as
wood finishes, may be obtained by addition of acid hardeners.
Urea resins find extensive use in textile finishing. For example,
cellulose fabrics may be padded into aqueous solutions of hydroxymethyl
ureas or their methyl ethers. Excess material is removed and the resins are
hardened in situ, using metal salt catalysts, by passing the fabric through
ovens at 130-160°C. Although there is negligible difference in the
appearance of the fabric, a considerable measure of crease resistance is
acquired. Such resin treatment does, however, lead to two immediate
problems. Firstly the cellulose fabric has lower tear and tensile strengths.
This problem is partially overcome by mercerization (steeping in sodium
hydroxide solution) before resin treatment. The second problem occurs
where the fabric is subjected to repeated bleaching action since the resin
reacts with hypochlorite bleach to give chloramines, which break down on
ironing, forming hydrochloric acid, which tenderizes the fabric. This
problem has been progressively reduced in recent years by the use of cyclic
urea derivatives which do not form chloramines.
1.3.2 Melamine-Formaldehyde Resins
Melamine (1,3,5-triamino-2,4,6-triazine) was first prepared by
Liebig in 1835. For a hundred years the material remained no more than a
laboratory curiosity until Henkel patented the production of resins by
condensation with formaldehyde. Today large quantities of melamineformaldehyde resins are used in the manufacture of moulding
compositions, laminates, adhesives, surface coatings and other
74
applications. Although in many respects superior in properties to the urea-
+
6CO(NH
2)2
6NH
3
300-6000C
400-600 atm
3CO
2
+ C3H6N
6
Melamine
+
C
3
H
6N6
Melamine
HCN + NH
4
Br
CNBr
H2+ NH
3
CNBr + 2NH
3
CNNH
2
+ NH
4
Br
Solvent
3CNNH
2
NH
3
CaCN
2
Acid
NH
2
CN
Cyanamide
70-80
0
C
NH
2
CNHCN
NH
Dicyanodiamid
based resins they are also significantly more expensive.
Melamine
A number of methods of producing melamine include:
(1) Heating dicyanodiamide, either with ammonia or on its own
under pressure.
(2) Fusion of dicyanodiamide with a guanidine salt.
(3) From urea.
(4) Electrolysis of dilute solutions of hydrogen cyanide in
ammonium bromide to give cyanogen bromide. This is then dissolved in a
solvent such as tetrahydrofuran and reacted with gaseous ammonia to
produce cyanamide. The cyanamide is then heated in an autoclave at about
190-200°C in the presence of ammonia and the melamine, recovered by
filtration.
Of these methods the first named was for many years the most
important commercially. Dicyanodiamide (“dicy”) is prepared by heating
cyanamide solution at 70-80°C. The cyanamide itself is prepared from
calcium cyanamide:
If “dicy” is heated just above its melting point of 209°C there is a
vigorous exothermic reaction results in the evolution of ammonia and the
formation of some melamine together with a number of complex waterinsoluble de-ammoniation products. In order to achieve a high yield of
melamine in commercial manufacture the reaction is carried out in the
presence of ammonia at about 300°C under pressure:
75
Melamine
NH
2
CNHCN
NH
N
C
N
C
N
C
NH
2
H2N
NH
2
N
C
N
C
N
C
NH
2
H2N
NH
2
NH
2
CO
NH
2
6
NH
3
-3H2O
NH
2
C
3
N
+
NH
2
CO
ONH
4
3
It will be noted that dicyanodiamide is the dimer of cyanamide and
melamine is the trimer.
Today melamine is usually prepared from urea. The detailed
mechanism of this reaction is believed to be as follows:
Only one melamine molecule is formed from six urea molecules,
whilst three molecules of ammonia carbamate are formed. Whilst this can be
recycled to urea the conversion from urea to melamine per cycle is at most
35%. Both the main route and the recycling operation involve high pressures
and the low process efficiency offsets some of the apparent economic
attractions of the route compared to those from “dicy”.
Melamine, a non-hygroscopic, white crystalline solid, melts with
decomposition above 347°C and sublimes at temperatures below the melting
point. It is only slightl y solubl e in wa ter; 10 0 ml of wa ter di ssol ve 0.38 g at
20°C and 3.7 g at 90°C. It is weakly basic and forms well-defined salts with
acids.
Resinification
100°C leads to the production of mixture of water-soluble
methylolmelamines. These hydroxy-methyl derivatives can possess up to
six methylol groups per molecule and include trimethylolmelamine and
Reaction of melamine with neutralized formaldehyde at about 80-
76
hexamethylolmelamine (Figure 26). The methylol content of the mixture
+
NHCH
2
OH HOCH
2
NH
NHCH
2
OCH
2
NH
+
H
2
O
N
C
N
C
N
C
CH2OH
CH
2
OH
HOH
2
C
Trimethylolmelamine
HOH
2
C
CH
2
OH
CH
2
OH
N
C
N
C
N
C
NH
NH
N
N
CH
2
OH
N
HOH2C
HN
HOH
2
C
Hexamethylolmelamine
NHCH2OH H
2
N
NHCH2NH
+ H
2
O
NHCH
2
OCH2NH
NHCH2NH
+ CH
2
O
will depend on the melamine formaldehyde ratio and on the reaction
conditions.
On further heating the methylolmelamines condense and a point is
reached where hydrophobic resin separates out on cooling. The
resinification is strongly dependent on the pH and is at a minimum at
about pH 10.0-10.5. An increase or decrease of pH from this value will
result in a considerable increase in resinifi ca tion rates.
There is some evidence that the principal resinification reaction
involves methylol-methylol condensations.
Figure 26
Methylene links may also be formed by the following reactions
In commercial practice the resin is condensed to a point close to the
hydrophobe point and then either applied to the substrate, or converted into
a moulding powder, before proceeding with the final cure.
In a typical process a jacketed still fitted with a stirrer and reflux
condenser in charged with 240 parts 37% w/w (40% w/v) formalin and the
pH adjusted to 8.0-8.5 using sodium carbonate solution with the aid of a pH
meter. One hundred and twenty six parts of melamine (to give a melamine
formaldehyde ratio of 1:3) are charged into the still and the temperature
raised to 85°C. The melamine goes into solution and forms methylol
derivatives. For treatment of fabrics, paper and leather this product may be
diluted and cooled for immediate use. It may also be spray dried to give a
more stable product. Cooling the solution would yield crystalline
trimethylolmelamine, which may be air dried but which is less soluble in
water than the spray-dried product.
77
For laminating and other purposes the initial product is further
heated to about 85°C with continuous stirring. After about 30 minutes, and
at regular intervals thereafter, samples of the resin are taken and added to
ice-cold water. Diminished water tolerance is indicated when the resin
solution becomes cloudy on entering the water. Reaction is then continued
until the stage is reached when addition of 3 cm3 of water will cause 1 cm3
of resin to become turbid.
Reactions may be carried out at lower pH values and higher
temperatures in order to achieve faster reactions. For some applications
where a high degree of fibre impregnation is required, hydrophilic resin
may also be produced. Such hydrophilic resins have limited stability in
aqueous solution and must either be used within a few hours of
manufacture or spray dried.
The more hydrophobic resins have only a slightly greater stability
in solution with a shelf-life of just a few days. Some improvement may be
achieved by diluting the resin content down to about 50% solids content
with industrial methylated spirit. The diluted resin should then be adjusted
to a pH of 9.0-9.5 to improve the stability. The addition of about 0.1%
borax (anhydrous) calculated on the weight of the solids content is useful in
obtaining this pH and maintaining it for several months. It is conveniently
added as an aqueous solution. The stabilized resins should be stored at 2035°C. Too low a storage temperature will cause precipitation, too high a
temperature, gelation. Precipitation may also occur it the resin is
insufficiently condensed, and gelation with over-condensation.
Moulding powders
Melamine-formaldehyde moulding powders are generally prepa red
by methods similar to those used with urea-formaldehyde material. In a
typical process an aqueous syrup, containing resin with a melamineformaldehyde molar ratio of 1:2, is compounded with fillers, pigments,
lubricants, stabilizers and in some cases accelerators in a dough-type mixer.
The product is then dried. Magnesium carbonate is employed to act as a pH
stabilizer during storage. For the more common decorative moulding
powders α-cellulose is used as a filler. Some bleached woodflour is
sometimes added to reduce shrinkage cracks near inserts. Because of the
high refractive index of the cured resin (~1.65) it is not possible to obtain
highly translucent mouldings using regenerated cellulose fillers as is
possible with U–F moulding powders.
78
Industrial grade materials employ fillers such as asbestos, silica
and glass fibre. These are incorporated by dry-blending methods similar to
those used with woodflour-filled phenolic compositions.
Mouldings from melamine-formaldehyde powders are superior to
the urea-formaldehyde plastics in a number of respects.
These include:
(1) Lower water absorption, especially with mineral-filled resins.
(2) Better resistance to staining by aqueous solutions such as fruit
juices and beverages. Further improvement in this respect is still desirable
and somewhat better results are claimed using benzoguanamine with the
melamine.
(3) Electrical properties, which are initially similar to those of
urea-formaldehyde resins, are maintained better in damp conditions and at
elevated temperatures.
(4) Better heat resistance.
(5) Greater hardness.
Compared with the phenolic resins they have a better colour range,
track resistance and scratch resistance. They have a similar order of heat
resistance, although their dimensional stability when exposed to hot dry
conditions is not so good. Melamine-formaldehyde moulding materials are
more expensive than general purpose urea-formaldehyde and phenolformaldehyde resins.
For high-duty electrical applications the mineral-filled melaminebased compositions have superior electrical insulation and heat resistance
to the cellulose-filled grades. The use of glass fibre leads to mouldings of
higher mechanical strength, improved dimensional stability and higher heat
resistance than with the fillers. Mineral-filled melamine-based powders
may be used when phenolics and urea-formaldehyde compositions are
unsuitable. They are thus to some extent competitive with the melaminephenolics, the alkyd moulding powders and, to some small extent, epoxy
moulding materials. It is therefore not surprising to find that usage of
mineral-filled M-F moulding powder is currently very small.
An interesting use of melamine resins in compression moulding
involves decorative foils. A suitably printed or decorated grade of paper is
impregnated with resin and dried. A compression moulding is then
prepared using a melamine-formaldehyde, or some other moulding
powder. Shortly before the cure is complete the mould is opened, the foil
placed in position and the resin in the foil cured in the position so that the
foil actually bonds on to the moulding.
79
Melamine-based compositions are easily moulded in conventional
compression and transfer-moulding equipment. Moulding temperatures are
usually in the range 145-165°C and moulding pressures 30-60 MPa. In
transfer moulding pressures of 75-150MPa are used. An 1/8 in thick
moulding required about 2.5 minutes cure at 150°C but shorter times are
possible with preheated powder.
The injection moulding of melamine-formaldehyde moulding
powders is now carried out on a small scale. Temperatures are somewhat
higher than for U-F (e.g. barrel temperatures 100-115°C; mould
temperatures 163-177°C). Otherwise the considerations are the same as for
the urea-formaldehyde compositions.
The principal application of melamine-formaldehyde moulding
compositions is for the manufacture of tableware, largely because of their
wide colour range, surface hardness and stain resistance. Cellulose-filled
compositions also find a small outlet for trays, clock cases and radio
cabinets and other purposes. The mineral-filled powders are used in
electrical applications and knobs and handles for kitchen utensils.
As with the U-F moulding powders the relative importance of M-F
moulding powders for other plastics materials and also in other uses for
melamine-formaldehyde resins has declined.
Laminates containing melamine-formaldehyde res i n
The high hardness, good scratch resistance, freedom from colour
and heat resistance of melamine-formaldehyde resins suggest possible use
in laminating applications. The use of laminates prepared using only
melamine resins as the bonding agent is, however, limited to some
electrical applications because of the comparatively high cost of the resin
compared with that of P-F resins. On the other hand a very large quantity
of decorative laminates are produced in which the surface layers are
impregnated with melamine resins and the base layers with phenolic resins.
Resins for this purpose generally use melamine-formaldehyde
ratios of 1:2.2 to 1:3. Where electrical grade laminates are required the
condensing catalyst employed is triethanolamine instead of sodium
carbonate.
Decorative laminates have achieved remarkable success because of
their heat resistance, scratch resistance and solvent resistance. Their
availability in a wide range of colours has led to their well-known
applications in table tops and as a wall-cladding in public buildings and
public transport vehicles.
80
Miscellaneous applications
In addition to their use in moulding powders and laminates,
melamine-formaldehyde resins are widely used in many forms.
Hot setting adhesives, prepared in the same way as laminating
resins, give colourless glue lines and are resistant to boiling water. Their
use alone has been limited because of high cost but useful products may be
made by using them in conjunction with a urea-based resin or with
cheapening extenders such as starch or flour.
Melamine is now widely used in conjunction with urea (and
formaldehyde) to produce adhesives of good strength, reactivity and water
resistance but with low ratios of formaldehyde to amine (i.e. urea and
melamine).
Melamine-formaldehyde condensates are also useful in textile
finishing. For example, they are useful agents for permanent glazing, rot
proofing, wool shrinkage control and, in conjunction with phosphorus
compounds, flame-proofing.
Modified melamine resins are also employed commercially.
Alkylated resins analogous to the alkylated urea-formaldehyde resins
provide superior coatings but are more expensive than the urea-based
products.
Treatment of hexahydroxymethylmelamine with an excess of
methanol under acid conditions yields the hexamethyl ether of
hexahydroxymethylmelamine (HHMM). Not only will this material
condense with itself in the presence of a strong acid catalyst to form
thermoset structures but in addition it may be used as a cross-linking a gent
in many polymer systems. Such polymers require an active hydrogen atom
such as in a hydroxyl group and cross-linking occurs by a trans-
etherification mechanism. Typical polymers are the acrylics, alkyds and
epoxides, HHMM having been particularly recommended in water-based
coating resins.
Paper with enhanced wet-strength may be obtained by
incorporating melamine resin acid colloid into the pulp. Melamine resin
acid colloid is obtained by dissolving a lightly condensed melamine resin
or trihydroxymethylmelamine, which are both normally basic in nature, in
dilute hydrochloric acid. Further condensation occurs in solution and
eventually a colloidal solution is formed in which the particles have a
positive charge. Careful control over the constitution of the colloidal
solution must be exercised in order to obtain products of maximum
stability.