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Файл:Plastics technology. Часть 2. Учебное пособие.pdf
41
formalin has a higher reactivity. Where a greater storage life is required the
+
OH
HCHO
OH
CH
2
OH
and
OH
CH
2
OH
formalin employed has a higher methanol content, but the resulting
increasing stability is at the expense of reduced reactivity.
Chemical aspects
Although phenolic resins have been known and widely utilized for
over 60 years their detailed chemical structure remains to be established. It
is now known that the resins are very complex and that the various
structures present will depend on the ratio of phenol to formaldehyde
employed, the pH of the reaction mixture and the temperature of the
reaction.
Reaction of phenol with formaldehyde involves a condensation
reaction which leads, under appropriate conditions, to a cross-linked
polymer structure. For commercial application it is necessary first to
produce a tractable fusible low molecular weight polymer which may,
when desired, be transformed into the cross-linked polymer. For example,
in the manufacture of a phenolic (phenol-formaldehyde, P-F) moulding a
low molecular weight resin is made by condensation of phenol and
formaldehyde. This resin is then compounded with other ingredients, the
mixture ground to a powder and the product heated under pressure in a
mould. On heating, the resin melts and under pressure flows in the mould.
At the same time further chemical reaction occurs, leading to cross-linking.
It is obviously desirable to process under such conditions that the required
amount of flow has occurred before the resin hardens.
The initial phenol-formaldehyde reaction products may be of two
types, novolaks and resols.
Novolaks
The novolaks are prepared by reacting phenol with formaldehyde in
a molar ratio of approximately 1:0.8 under acidic conditions. Under these
conditions there is a slow reaction of the two reactants to form the o- and p-
hydroxymethylphenols:
These then condensate rapidly to form products of the
42
bis(hydroxyphenyl)-methane (HPM) type:
+
OH
OH
CH2OH
OH
CH
2
OH
OH
CH
2
OH
2,2, HPM
HO
CH
2
OH
2,4,HPM
HO CH
2
OH
4,4, HPM
HO CH
2
OH
CH
2
OH
CH
2
CH
3
CH
2
OH
There are three possible isomers and the proportions in which they
are formed will depend on the pH of the reaction medium. Under the acid
conditions normally employed in novolak manufacture the 2,4'- and 4,4'HPM compounds are the main products:
These materials will then slowly react with further formaldehyde to
form their own methylol derivatives which in turn rapidly react with further
phenol to produce higher polynuclear phenols. Because of the excess of
phenol there is a limit to the molecular weight of the product produced, but
on average there are 5-6 benzene rings per molecule. A typical example of
the many possible structures is shown in Figure 18.
The novolak resins themselves contain no reactive methylol groups
and do not form cross-linked structures on heating. If, however, they are
mixed with compounds capable of forming methylene bridges, e.g.
hexamethylenetetramine or paraformaldehyde, they cross-link on heating to
form infusible, “thermoset” structures.
Figure 18
43
In general it is considered essential that the bulk of the phenol used
HOCH
2
OH
CH
2
CH
2
OH
CH
2
OH
O
CH
2
OH
CH
2
OH
initially should not be substituted, i.e. should be reactive, at the o- and p-
positions and is thus trifunctional with respect to the reaction with
formaldehyde.
Resols
A resol is produced by reacting a phenol with an excess of
aldehyde under basic conditions.
In this case the formation of phenol-alcohols is rapid but their
subsequent condensation is slow. Thus there is a tendency for polyalcohols,
as well as monoalcohols, to be formed. The resulting polynuclear
polyalcohols are of low molecular weight. Liquid resols have an average of
less than two benzene rings per molecule, while a solid resol may have only
three to four. A typical resol would have the structure shown in Figure 19.
Figure 19
Heating of these resins will result in cross-linking via the
uncondensed methylol groups or by more complex mechanisms. The resols
are sometimes referred to as one-stage resins since cross-linked products
may be made from the initial reaction mixture solely by adjusting the pH.
On the other hand the novolaks are sometimes referred to as two-stage
resins as here it is necessary to add some agent which will enable additional
methylene bridges to be formed.
Hardening
The novolaks and resols are soluble and fusible low molecular
weight products. They were referred to by Baekeland as A-stage resins. On
hardening, these resins pass through a rubbery stage in which they are
swollen, but not dissolved, by a variety of solvents. This is referred to as
the B-stage. Further reaction leads to rigid, insoluble, infusible, hard
products known as C-stage resins. When prepared from resols the B-stage
resin is sometimes known as a resitol and the C-stage product a resit. The
terms A-, B- and C-stage resins are also sometimes used to describe
analogous states in other thermosetting resins.
44
The mechanism of the hardening processes has been investigated
OH
CH2OH
OH
CH
2
OH
OH
CH2OCH
2
OH
OH
OH
CH
2
OH
CH
2
O
CH
2
O
O
CH
2
n
O
H
H
by Zinke in Austria, von Euler in Sweden and Hultzsch in Germany using
blocked methylol phenols so that only small isolable products would be
obtained.
In general their work indicates that at temperatures below 160°C
cross-linking occurs by phenol methylol-phenol methylol and phenol
methylol-phenol condensations, viz Figure 20.
Figure 20
As these condensation reactions can occur at the two ortho and the
para positions in phenol, m-cresol and 3,5-xylenol, cross-linked structures
will be formed.
Above 160°C it is believed that additional cross-linking reactions
take place involving the formation and reaction of quinone methides by
condensation of the ether linkages with the phenolic hydroxyl groups:
These quinone methide structures are capable of polymerization
and of other chemical reactions.
It is likely that the quinone methide and related structures formed at
these temperatures account for the dark colour of phenolic compression
mouldings. It is to be noted that cast phenol-formaldehyde resins, which are
hardened at much lower temperatures, are water-white in colour. If,
45
however, these castings are heated to about 180°C they darken
considerably.
Resin manufacture
Both novolaks and resols are prepared in similar equipment, shown
dia-grammatically in Figure 21. The resin kettle may be constructed from
copper, nickel or stainless steel where novolaks are being manufactured.
Stainless steel may also be used for resols but where colour formation is
unimportant the cheaper mild steel may be used.
In the manufacture of novolaks, 1 mole of phenol is reacted with
about 0.8 mole of formaldehyde (added as 37% w/w formalin) in the
presence of some acid as catalyst. A typical charge ratio would be:
Phenol 100 parts by weight
Formalin (37% w/w) 70 parts by weight
Oxalic acid 1.5 parts by weight
Figure 21 – Diagrammatic repre sen tat ion of res in kettle and assoc iated
equipment used for the preparation of phenolic resins.
46
The reaction mixture is heated and allowed to reflux, under
atmospheric pressure at about 100°C. At this stage valve A is open and
valve В is closed. Because the reaction is strongly exothermic initially it
may be necessary to use cooling water in the jacket at this stage. The
condensation reaction will take a number of hours, e.g. 2-4 hours, since
under the acidic conditions the formation of phenol-alcohols is rather slow.
When the resin separates from the aqueous phase and the resin reaches the
requisite degree of condensation, as indicated by refractive index
measurements, the valves are changed over (i.e. valve A is closed and valve
В opened) and water present is distilled off.
In the case of novolak resins the distillation is normally carried out
without the application of vacuum. Thus, as the reaction proceeds and the
water is driven off, there is a rise in the temperature of the resin which may
reach as high as 160°C at the end of the reaction. At these temperatures the
fluid is less viscous and more easily stirred. In cases where it is important
to remove the volatiles present, a vacuum may be employed after the
reaction has been completed, but for fast-curing systems some of the
volatile matter (mainly low molecular weight phenolic bodies) may be
retained.
The end point may be checked by noting the extent of flow of a
heated pellet down a given slope or by melting point measurements. Other
control tests include alcohol solubility, free phenol content and gelation
time with 10% hexa.
In the manufacture of resols a molar excess of formaldehyde (1.5-
2.0:1) is reacted with the phenol in alkaline conditions. In these conditions
the formation of the phenol alcohols is quite rapid and the condensation to a
resol may take less than an hour. A typical charge for a laboratory-scale
preparation would be:
Phenol 94 g (1 mole)
Formalin (40%) 112 cm3 (1.5 moles formaldehyde)
0.88 ammonia 4 cm3
The mixture is refluxed until the reaction has proceeded
sufficiently. It may then be neutralized and the water formed distilled off,
usually under reduced pressure to prevent heat-hardening of the resin.
Because of the presence of hydroxymethyl groups the resol has a greater
water-tolerance than the novolak.
The reaction may be followed by such tests as melting point,
acetone or alcohol solubility, free phenol content or loss in weight on
stoving at 135°C.
47
Two classes of resol are generally distinguished, water-soluble
resins prepared using caustic soda as catalyst, and spirit-soluble resins
which are catalyzed by addition of ammonia. The water-soluble resins are
usually only partially dehydrated during manufacture to give an aqueous
resin solution with a solids content of about 70%. The solution viscosity
can critically affect the success in a given application. Water-soluble resols
are used mainly for mechanical grade paper and cloth laminates and in
decorative laminates.
In contrast to the caustic soda-catalyzed resols the spirit-soluble
resins have good electrical insulation properties. In order to obtain superior
insulation characteristics a cresol-based resol is generally used. In a typical
reaction the refluxing time is about 30 minutes followed by dehydration
under vacuum for periods up to 4 hours.
Moulding powders
Novolaks are most commonly used in the manufacture of moulding
powders although resols may be used for special purposes such as in
minimum odour grades and for improved alkali resistance. The resins ar e
generally based on phenol since they give products with the greatest
mechanical strength and speed of cure, but cresols may be used in acidresisting compounds and phenol-cresol mixtures in cheaper compositions.
Xylenols are occasionally used for improv ed alkali resi stan ce.
The resols may be hardened by heating and/or by addition of
catalysts. Hardening of the novolaks may be brought about by addition of
hexamethylenetetramine (hexa, hexamine). Because of the exothermic
reaction on hardening (cure) and the accompanying shrinkage, it is
necessary to incorporate inert materials (fillers) to reduce the resin content.
Fillers are thus generally necessary to produce useful mouldings and are not
incorporated simply to reduce cost. Fillers may give additional benefits
such as improving the shock resistance.
Other ingredients may be added to prevent sticking to moulds
(lubricants), to promote the curing reaction (accelerators), to improve the
flow properties (plasticizers) and to colour the product (pigments).
Compounding ingredients
It is thus seen that a phenol-formaldehyde moulding powder will
contain the follo wing ingred ien ts:
(1) Resin.
(2) Hardener (with Novolaks).
48
(3) Accelerator.
+
6CH
2
O
4NH
3
CH
2
N
CH
2
N
CH
2
N
CH
2
N
CH
2
CH
2
(4) Filler.
(5) Lubricant.
(6) Pigment.
(7) Plasticizer (not always used).
In addition to the selection of phenol used and the choice between
novolak and resol there is a number of further variations possible in the
resin used. For example, in the manufacture of a novolak resin slight
adjustment of phenol/formaldehyde ratio will affect the size of novolak
molecule produced. Higher molecular weight novolaks give a stiff-flow
moulding powder but the resin being of lower reactivity, the powders have
a longer cure time. A second variable is the residual volatile content. The
greater the residual volatiles (phenolic bodies) the faster the cure. Thus a
fast-curing, stiff-flow resin may be obtained by using a
phenol/formaldehyde ratio leading to larger molecules and leaving some of
the low molecular weight constituents in the reaction mixture. Yet another
modification may be achieved by changing the catalyst used. Thus whereas
in the normal processes, using oxalic acid catalysts, the initial products are
p-p- and o-p-diphenylmethanes, under other conditions it is possible to
achieve products which have reacted more commonly in the ortho-position.
Such resins thus have the p-position free and, since this is very reactive to
hexa, a fast-curing resin is obtained.
Hexa is used almost universally as the hardener. It is made by
passing a slight excess of ammonia through a lightly stabilized aqueous
solution of formaldehyde, concentrating the liquor formed and crystallizing
out the hexa.
Between 10 and 15 parts of hexa are used in typical moulding
compositions. The mechanism by which it cross-links novolak resins is not
fully understood but it appears capable of supplying the requisite methylene
bridges required for cross-linking. It also functions as a promoter for the
hardening reaction.
Basic materials such as lime or magnesium oxide increase the
49
hardening rate of novolak-hexa compositions and are sometimes referred to
as accelerators. They also function as neutralizing agents for free phenols
and other acidic bodies which cause sticking to, and staining of, moulds
and compounding equipment. Such basic substances also act as hardeners
for resol-based compositions.
Woodflour, a fine sawdust preferably obtained from softwoods
such as pine, spruce and poplar, is the most commonly used filler.
Somewhat fibrous in nature, it is not only an effective diluent for the resin
to reduce exotherm and shrinkage, but it is also cheap and improves the
impact strength of the mouldings. There is a good adhesion between
phenol-formaldehyde resin and the woodflour and it is possible that some
chemical bonding may occur.
Another commonly employed low-cost organic filler is coconut
shell flour. This can be incorporated into the moulding composition in large
quantities and this results in cheaper mixes than when woodflour is used.
The mouldings also have a good finish. However, coconut shell flour-filled
mouldings have poor mechanical properties and hence the filler is generally
used in conjunction with woodflour.
For better impact strength cotton flock, chopped fabric or even
twisted cord and strings may be incorporated. The cotton flock-filled
compounds have the greatest mouldability but the lowest shock resistance
whilst the twisted cords and strings have the opposite effect. Nylon fibres
and fabrics are sometimes used to confer strength and flexibility and glass
fibres may be used for strength and rigidity.
Asbestos may be used for improved heat and chemical resistance
and silica, mica and china clay for low water absorption grades. Iron-free
mica powder is particularly useful where the best possible electrical
insulation characteristics are required but because of the poor adhesion of
resin to the mica it is usually used in conjunction with a fibrous material
such as asbestos. Organic fillers are commonly used in a weight ratio of 1:1
with the resin and mineral fillers in the ratio 1.5:1.
In some countries the extensive use of asbestos as a filler is
somewhat discouraged because of the hazards associated with its use. In
other parts of the world moulding compositions of enhanced heat resistance
have been developed by the use of especially heat-resisting polymers used
in conjunction with asbestos and other mineral fillers.
Stearic acid and metal stearates such as calcium stearate are
generally used as lubricants at a rate of about 1-3% on the total compound.
Waxes such as carnauba and ceresin or oils such as castor oil may also be
used for this purpose.
50
In order that the rate of cure of phenolic moulding compositions is
sufficiently rapid to be economically attractive, curing is carried out at a
temperature which leads to the formation of quinone methides and their
derivatives which impart a dark colour to the resin. Thus the range of
pigments available is limited to blacks, browns and relatively dark blues,
greens, reds and oranges.
In some moulding compositions other special purpose ingredients
may be incorporated. For example, naphthalene, furfural and dibutyl
phthalate are occasionally used as plasticizers or more strictly as flow
promoters. They are particularly useful where powders with a low
moulding shrinkage are required. In such formulations a highly condensed
resin is used so that there will be less reaction, and hence less shrinkage,
during cure. The plasticizer is incorporated to the extent of about 1% to
give these somewhat intractable materials adequate flow properties.
Compounding of phenol-formaldehyde moulding compositions
Although there are many variants in the process used for
manufacturing moulding powders, they may conveniently be classified into
dry processes and wet processes.
In a typical dry process, finely ground resin is mixed with the other
ingredients for about 15 minutes in a powder blender. This blend is then fed
on to a heated two-roll mill. The resin melts and the powdery mix is fluxed
into a leathery hide which bands round the front roll. The temperatures
chosen are such that the front roll is sufficiently hot to make the resin tacky
and the real roll somewhat hotter so that the resin will melt and be less
tacky. Typical temperatures are 70-100°C for the front roll and 100-120°C
for the back. As some further reaction takes place on the mill, resulting in a
change of melting characteristics, the roll temperatures should be carefully
selected for the resin used. In some processes two mills may be used in
series with different roll temperatures to allow greater flexibility in
operation. To achieve consistency in the end-product a fixed mixing
schedule must be closely followed. Milling times vary from 10 minutes
down to a straight pass through the mill.
The hide from the mill is then cooled, pulverized with a hammermill and the resulting granules are sieved. In a typical general purpose
composition the granules should pass a 14x26 sieve. For powders to be
used in automatic moulding plant fine particles are undesirable and so
particles passing a 100x41 sieve (in a typical process) are removed. In