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Файл:Plastics technology. Часть 2. Учебное пособие.pdf
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a pH of 4-5 during distillation and hardening leads to a hardening rate
sufficiently slow for the distillation of the water to be carried out without
gelling in the reaction vessel, but not such that final hardening takes an
infinitely slow time. Even so cure times are very long. It is not desirable to
exceed cure temperatures of 75-80°C as this would lead to vaporization of
occluded water with subsequent void formation and blistering. Cast
phenolic resins have been used for umbrella handles, knobs, propelling
pencil bodies and for other purposes where attractive appearance is of
importance.
There has b een so me interes t in phenolic resin foams during recent
years but these have yet to achieve large-scale usage. They could, however,
become important in the event of legislation restricting the use of
polyurethanes. These foams are self-extinguishing but currently more
expensive to produce than the well-established expanded polystyrene. For
good control of properties mechanical mixing devices, similar to those
employed with polyurethanes, are used.
Somewhat intermediate in nature between the moulded powders
and the laminates are the fibre-resin preform mouldings. These are
produced by making a resin-containing fibre preform, usually of the same
shape as the finished moulding, and then subjecting the preform to a
consolidating pressure in a compression mould.
Preform mouldings are particularly useful in carrying containers
and protective covers. Examples of their use include television receiver
backs, moulded suitcases and typewriter cases. Although the finish
obtained during moulding is frequently adequate in industrial applications
some improvement is necessary where a good appearance is desired.
Methods used include painting or vacuum forming a thermoplastics sheet
material over the outside of a moulded preform which has been coated with
a suitable adhesive.
Phenolic resins are useful surface coating materials. Resols are
useful for stoving lacquers for coating chemical plant, textile equipment,
razor blades, brassware and food cans. Phenolic resins are used with
poly(vinyl formal) as a flexible, tough and solvent-resistant wire enamel.
Oil-soluble resins based on synthetic phenols form the basis of some gloss
paints.
A variety of adhesives based on phenolic resins are available.
These include metal cements made by combining a resol with a vinyl
polymer such as poly(vinyl formal). Resols are also used for plywood
glues, which may be cured using alkaline catalysts at 135°C. If resorcinol
62
and/or paraformaldehyde are included in the formulation, slightly lower
curing temperatures may be used. These glues have good resistance to
aging, moisture and bacteria. Highly-filled novolak-hexa compositions
form the basis of lamp capping cements.
Other uses include impregnation of wood to improve dimensional
stability and reduce water absorption, sealing of porous metal castings by
impregnation, and coil impregnation, to give a rigid structure both heat and
water resistant.
Resorcinol-formaldehyde adhesives
Resorcinol (1.3-dihydroxybenzene) is more reactive to
formaldehyde than phenol itself, the two hydroxyl groups reinforcing each
other in activating the o-and p-positions. This allows faster reactions to
occur than with phenol, when compared at the same temperature, and under
appropriate conditions curing can occur at normal room temperatures.
Reaction rates are at a minimum at pH 3, and, unlike with the
phenol-formaldehyde condensates, which have a minimum at pH 7, setting
can occur under neutral conditions.
In commercial practice resorcinol is seldom used on its own but in
conjunction with phenol when condensing with formaldehyde.
Resorcinol-formaldehyde (RF) resins do not find use in
conventional plastics applications but as reactive adhesives. Their major
use is in the manufacture of glued timber structures, where the ability to
harden the polymer under neutral conditions (i.e. under non-acid conditions
which thus does not damage the wood) and, if desired, at room temperature
is a particular advantage. The bonds are very durable and “weatherproof”
and, like PF's, are resistant to insects, fungi and other micro-organisms.
They also have a long storage life. Their main limitations are their cost and
their naturally dark red colour, although this can on occasion be used to
some effect decoratively.
The adhesive resins are of the novolak type and generally hardened
by paraformaldehyde (which may be supplied mixed with coconut shell
flour or diatomaceous earth) in conjunction with an accelerator such as
magnesium oxide.
1.3 Aminoplastics
The term aminoplastics has been coined to cover a range of
resinous polymers produced by interaction of amines or amides with
aldehydes. Of the various polymers of this type that have been produced
63
there are two of current commercial importance in the field of plastics, the
urea-formaldehyde and the melamine-formaldehyde resins. There has in
the past also been some commercial interest in aniline-formaldehyde resins
and in systems containing thiourea but today these are of little or no
importance. Melamine-phenol-formaldehyde resins have also been
introduced for use in moulding powders, and benzoguanamine-based resins
are used for surface coating applications.
Interest in aminoplastics dates from the publication of a patent by
John in 1918 which disclosed resinous materials prepared by heating urea
with commercial formalin and which suggested the use of the resultant
viscous solutions as adhesives and as impregnants for fabrics. In the
following years patents were taken out by Pollak and Ripper, and
Goldschmidt and Neuss, the former pair directing their efforts towards the
manufacture of an “organic glass”, the latter group towards moulding
compositions. In 1926, as a result of work by E.C. Rossiter, moulding
powders based on urea-thiourea-formaldehyde were marketed under the
trade name Beetle by The British Cyanides Co. Ltd (later known as British
Industrial Plastics Ltd). Similar products were subsequently produced in
other countries.
During the next 15 years the urea resins were also developed for
use as adhesives, as textile finishing agents and in the production of surface
coatings and wet-strength paper. Since World War II the development of
chipboard has resulted in a large new outlet for urea-based resins which
have also found other uses, such as in firelighters and foams.
In 1935 Henkel patented the production of resins based on
melamine. Today these resins are important in the manufacture of
decorative laminates and in tableware.
By the mid-1990s world production of aminoplastics was estimated
at about 6000000 t.p.a. of which more than 5000000 t.p.a. were ureaformaldehyde resins. The bulk of the rest were melamine-formaldehyde.
Such bald statistics, however, disguise the fact that a considerable amount
of aminoplastics used are actually co-condensates of urea, melamine and
formaldehyde.
By far the bulk of amino resins are used in the woodworking
industry for the manufacture of chipboard, plywood and as general glues
and adhesives. Melamine-formaldehyde is an important component of
decorative laminates. The amount of amino resins used for moulding
applications is only of the order of 5% of the total.
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1.3.1 Urea-Formaldehyde Resins
2NH
3
+CO
2
NH
2
CO
2NH4
CO(NH2)2 + H2O
C
NH
2
NH
2
O
HCHO
2HCHO
C
NHCH
2
OH
NH
2
O
C
NHCH
2
OH
NHCH2OH
O
Of the various amino-resins that have been prepared, the urea-
formaldehyde (U-F) resins are by far the most important commercially.
Like the phenolic resins, they are, in the finished product, cross-linked
(thermoset) insoluble, infusible materials. For application, a low molecular
weight product or resin is first produced and this is then cross-linked only
at the end of the fabrication process.
In a general comparison with phenolic resins, the U-F materials are
cheaper, light in colour, are lacking in odour, have better resistance to
electrical tracking but have an inferior heat resistance and a higher water
absorption.
Raw materials
Urea is a white crystalline compound with a melting point of
132.6°C and is highly soluble in water. It is substantially cheaper than the
other intermediate (formaldehyde) used in the resin preparations.
Urea is prepared commercially by the reaction of liquid carbon
dioxide and ammonia in silver-lined autoclaves, at temperatures in the
range 135-195°C and pressure of 70-230 atm. The reaction proceeds by
way of ammonium carbamate:
A 40-60% conversion per pass is achieved and unreacted feedstock
are returned to the compressors.
Theories of resinification
Urea-formaldehyde resins are usually prepared by a two-stage
reaction. The first stage involves the reaction of urea and formaldehyde
under neutral or mildly alkaline conditions, leading to the production of
mono and dimethylol ureas. The ratio of mono to dimethylol compounds
will depend on the urea-to-formaldehyde ratio and it is important that there
should be enough formaldehyde to allow some dimethylol urea formation.
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If the product of the first stage, which in practice usually also
H2NCONHCH
2
OH + H
2
NCONHCH2OH
HNCONHCH2NHCONHCH
2
HO(CH2NHCONH)nCH
2
OH
contains unreacted urea and formaldehyde, is then subjected to acid
conditions at elevated temperatures the following sequence of events is
observed during the second stage:
(1) A solution is produced from which, if cooled, a white
precipitate would be obtained.
(2) As heating proceeds, the temperature at which precipitation
occurs drops progressively until a stage is reached when the condensation
products remain in solution at room temperature.
(3) With further heating there is an increase in viscosity and the
syrup sets to an insoluble and irreversible gel which eventually converts,
with the evolution of water and formaldehyde to a hard, colourless,
transparent and infusible mass.
For technical purposes it is often convenient to both arrest the
reaction prior to gelation by changing to a slightly alkaline pH and to
remove some if not all of the water. The first hardening reaction ma y then
be carried out when desired by changing once again to an acid pH.
The precise mechanisms involved during the second stage are not
fully understood although a number of theories have been proposed. With
the present state of knowledge it appears that in the first part of the secondstage methylol ureas condense with each other by reaction of an ~СН2ОН
group of one molecule with an ~NH2 group of another:
Since ~NH~ groups (as found in dimethylol urea) are apparently
less reactive the initial reaction products appear to be based on linear
polymers of the form
These probably form the basis of the amorphous precipitates
formed on cooling. The more soluble resins produced on continuation of
the reaction probably contain pendant methylol groups formed by reactions
of the ~NH~ groups with free formaldehyde (Figure 24 I).
These methylol groups and the methylol groups on the chain ends
of the initial reaction product can then react with other methylol groups to
give either linkages or with amine groups to give methylene linkages.
Furthermore the ether linkages on heating may break down to methylene
linkages with the evolution of formaldehyde (Figure 24 II and III).
66
NH
CH
2
O
CH
2
OH
N
NHCH
2
OH
HOCH
2
NH
NHCH
2
OCH
2
NH
[-CH2O]
NHCH
2
OH
H
2
N
NHCH
2
NH
I
II
III
Figure 24
When reactions II and III occur on average more than twice per
molecule the resin gels, and cross-linking may be considered to have
occurred.
U-F moulding mate r ia ls
Thermosetting compositions based on urea-formaldehyde are
widely employed because of their low cost, wide colour range, rigidity and
good electrical properties.
Manufacture
A moulding powder based on urea-formaldehyde will contain a
number of ingredients. Those most commonly employed include the
following:
(1) Resin.
(2) Filler.
(3) Pigment.
(4) Hardener (more commonly referred to as an accelerator).
(5) Stabilizer.
(6) Plasticizer.
(7) Lubricant.
The first stage of resin preparation is to dissolve urea into the 36%
w/w formalin which has been adjusted to a pH of 8 with caustic soda.
Since formaldehyde interferes with normal functioning of universal
indicator a pH meter is used when making pH adjustments. The blending
may be carried out without heating in a glass-lined or stainless-steel reactor
for about 90 minutes. In an alternative process the blending is carried out at
about 40°C for 30 minutes. In some cases the pH, which may drop during
reaction, is adjusted by addition of small quantities of hexamine. The ureaformaldehyde ratios normally employed are in the range 1:1.3 to 1:1.5.
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Only a small amount of reaction occurs in the first stage so that the solution
at the end of this process contains urea, formaldehyde, and mono- and
demethylol urea, the latter in insufficient concentration at this stage to
separate out.
Only a limited range of fillers is used commercially with U-F
resins. Bleached wood pulp is employed for the widest range of bright
colours and in slightly translucent mouldings. Woodflour, which is
significantly cheaper, may also be used. Unpigmented, this gives
mouldings brown in colour and somewhat translucent in thin sections. To
mask the brown colour heavy pigmentation is commonly employed and
this results in opaque mouldings. For mouldings of enhanced translucency,
chopped regenerated cellulose (Cellophane) film, which is free from voids
and has a refractive index (1.565) close to that of the resin (1.55-1.56) can
be incorporated. Fabric fillers and minerals fillers and not commonly
employed with U-F resins.
A wide variety of pigments is now used in U-F moulding
compositions. Their principal requirements are that they should not affect
the stability of moulding characteristics of the power, that they should be
stable to processing conditions, be unaffected by conditions of service,
including insolubility in any solvents with which the mouldings might
come into contact, be light fast, and not interfere with the electrical
properties.
In order to obtain a sufficient rate of cure at moulding temperatures
it is usual to add about 0.2-2.0% of a “hardener” (accelerator). This
functions by decomposing at moulding temperatures to give an acidic body
that will accelerate the cure rate. Some of the more prominent latent acids
are ammonium sulphamate, ammonium phenoxyacetate, ethylene sulphite
and trimethyl phosphate.
Urea-formaldehyde powders have a limited shelf-life but some
improvement is made by incorporating a stabilizer such as hexamine into
the moulding power. In some formulations the cure rate and the related
time for flow are controlled by keeping the latent acid catalyst fixed and
adjusting the stabilizer.
Plasticizers are used in special grades of moulding powders. Their
main virtue is that they enable more highly condensed resins to be used and
thus reduce curing shrinkage whilst maintaining good flow properties.
Glyceryl α-tolyl ether (monocresyl glycidyl ether) is often used for this
purpose. Plasticizers may also be used in small quantities to improve the
flow of other grades.
68
Metal stearates such as zinc, magnesium or aluminium stearates
are commonly used as lubricants at about 1% concentration. Other
materials that have been used successfully include oxidized paraffin wax
and sulphonated castor oil.
In typical manufacturing processes the freshly prepared ureaformaldehyde initial reaction product is mixed with the filler (usually with
a dry weight resin-filler ratio of about 2:1) and other ingredients except
pigment in a trough mixer. This process, which takes about two hours at
60°C, enables thorough impregnation of the wet base with the resin
solution and also advances the resinification reaction. After a check has
been made that it is slightly alkaline the resulting wet base is then fed to a
drier which may be either of the turbine or rotary type. The turbine drier
consists of a number of slowly rotating circular trays stacked one above the
other in a large oven. Each of the trays has a number of radial slits. The
powder is fed to the top tray where it rests for one revolution when, by
means of scraper blades, it is pushed through the slits on the second tray,
where the process is repeated. In a typical process the residence time of the
mixture in the drier is about two hours at 100°C. In an alternative process
the wet base is fed into a rotary drier in which it remains for about 3/4-1
hour whilst being subjected to counterblast air at 120-130°C. This process
reduces the water content from about 40% to about 6% and also advances
the condensation.
On emerging from the drier the base is hammer-milled and then
ball-milled for 6-9 hours. The pigments are added at the ball-mill stage.
During this process samples are taken and checked for colour and
processing characteristics. It is frequently necessary to make slight
adjustments to the formulation by adding further pigment or other
ingredients at this stage. The ball-milling process ensures a good
dispersion of pigment and gives a fine powder that will produce mouldings
of excellent fi nish. On the other hand the powder has a high bulk factor
and problems of air and gas trappings will occur during moulding. These
problems are overcome by densifying the product.
One method of densification is to heat the powder as it passes
along a belt and to drop the heated powder into the nip of a two-roll mill. In
this process the material passes directly through the rolls to form a strip
which is then hammer-milled to give powder whi ch is in the f orm of tiny
flat flakes. In another process the fine powder is slowly stirred in a large
pot and water, or a water-methanol blend, or steam, run into the mixture.
The particles partly cohere in a damp conditions and on subsequent drying
69
give densified granules. A third process is to charge the powder into an
internal mixer which is at a temperature about 100°C. The particles cohere
and after about two minutes the batch is fed to a hammermill to give a
coarse granule.
As an alternative to the wet process described above, moulding
compositions may be made by mixing a powdered resin or a methylol
derivative with other ingredients on a two-roll mill or in an internal mixer.
The condensation reaction proceeds during this process and when deemed
sufficiently advanced, the composition is sheeted off and disintegrated to
the desired particle size. This dry process is not known to be used in any
current commercial operation.
Control tests on the moulding powder include measurement of
water content, flow, powder density and rate of cure.
From the above discussion it will be recognized that in addition to
differences in colour, commercial urea-formaldehyde moulding powders
may differ in the following respects:
(1) The nature of the filler used.
(2) The ease of flow (dependent on the degree of heating during the
drying stages, and in some cases on the heating operations associated with
densifying).
(3) The speed of cure, partly related to the ease of flow but
associated with the amounts of hardener and stabilizer.
(4) The type of grind.
(5) The presence of absence of plasticizer.
It is these differences which determine the range of grades at
present commercially availab le.
Processing
Urea-formaldehyde moulding powders may be moulded without
difficulty on conventional compression and transfer moulding equipment.
The powders, however, have limited storage life. They should thus be
stored in a cool place and, where possible, used within a few months of
manufacture.
Properties and applications
When they were first introduced, the value of U-F moulding
powders lay in their availability in a wide range of colours, at that time a
novelty amongst thermosetting moulding composition. The wide colour
range possible continues to be a reason for the widespread use of the
70
material but other useful features have also become manifest.
The major desirable features of U-F mouldings are:
(1) Low cost. The cheaper grades are sometimes lower in weight
cost than the general purposes phenolics.
(2) Wide colour range.
(3) T hey do not impart taste and odour to foodstuffs and beverages
with which they come in contact.
(4) Good electrical insulation properties with particularly good
resistance to tracking.
(5) Resistance to continuous heat up to a temperature of 70°C.
Recent estimates suggest that in the early 1990s about 43% of U-F
moulding powders were used in electrical and electronic applications.
Another, more satisfactory, application area is in the sanitary sector, with
nearly 20% of the market, for such uses as toilet seats and miscellaneous
bathroom equipment.
At one time a major market was for bottle caps and closures, where
the low cost, wide colour range and freedom from taste and odour were
attractive features.
Miscellaneous uses include meat trays, toys, knobs, switches and
lampshades, U-F lampshades are generally strictly utilitarian in design and
of limited aesthetic appeal. It is important in this application to ensure
adequate ventilation of the air space above the lamp in order to prevent
overheating and subsequent cracking of the shade. For similar reasons
fittings for ceiling light bowls, as often used in bathrooms and kitchens,
may fail through lack of adequate ventilation.
U-F moulding materials are relatively much less important than
they were 30 years ago. Western European production in 1991 was
approximately 70000 tonnes, slightly greater than P-F moulding material
production. U-F resin used for this application is, however, probably less
than 5% of total U-F resin production.
Adhesives and related uses
By far t he bu lk o f U-F resins are used as adhesives for the particle
board, plywood and furniture industries.
To prepare a suitable resin, formalin is first neutralized to a pH of
7.5 and urea is then dissolved into it (U-F molar ratio 1:2). Sodium formate
may be added as a buffer to regulate the pH. The mixture is boiled under
reflux, typically for about 15 minutes, to give dimethylol urea and other
low molecular weight products. The resins in then acidified to pH 4,