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Файл:Plastics technology. Часть 2. Учебное пособие.pdf
81
1.3.3 Melamine-Phenolic Resins
Moulding powders based on melamine-phenol-formaldehyde
resins were introduced by Bakelite Ltd, in the early 1960s.
The principal characteristic of these materials is the wide range of
colours possible, including many intense bright colours. The melaminephenolics may be considered to be intermediate between the phenolic
moulding materials and those from melamine-formaldehyde. As a result
they have better moulding latitude and mouldings have better dry heat
dimensional stability than the melamine-formaldehyde materials. Their
tracking resistance is not as good as melamine-formaldehyde materials but
often adequate to pass tracking tests. The main applications of these
materials are as handles for saucepans, frying pans, steam irons and coffee
pots where there is a requirement for a coloured heat-resistant material. It
was never likely that the melamine-phenolics would absorb much of the
market held by melamine resins, irrespective of price, since this market is
largely dependent on either the non-odorous nature of the good tracking
resistance of the material used. Neither of these two requirements were
fulfilled by the melamine-phenolics. Future developments thus seem to lie
in the creation of new markets for a coloured, heat-resistant material
intermediate in price between the phenolic and melamine materials.
1.3.4 Aniline-Formaldehyde Resins
Although occasionally in demand because of their good electrical
insulation properties, aniline-formaldehyde resins are today only rarely
encountered. They may be employed in two ways, either as an unfilled
moulding material or in the manufacture of laminates.
To produce a moulding composition, aniline is first treated with
hydrochloric acid to produce water-soluble aniline hydrochloride. The
aniline hydrochloride solution is then run into a large wooden vat and
formaldehyde solution is run in at a slow but uniform rate, the whole mix
being subject to continuous agitation. Reaction occurs immediately to give
a deep orange-red product. The resin is still a water-soluble material and so
it is fed into a 10% caustic soda solution to react with the hydrochloride,
thus releasing the resin as a creamy yellow slurry. The slurry is washed
with a counter-current of fresh water, dried and ball-milled.
Because of the lack of solubility in the usual solvents, anilineformaldehyde laminates are made by a “pre-mix” method. In this process
the ani line h ydroc hloride -formaldehyde product is run into a bath of paper
pulp rather than of caustic soda. Soda is then added to precipitate the resin
82
on to the paper fibres. The pulp is then passed through a paper-making
NH
2
+ CH
2
O
H2N
CH
2
OH
HN
CH
2
+ CH
2
O
HN
CH
2
CH
2
HN
CH
2
CH
2
HN
CH
2
HN
+ H
2
O
machine to give a paper with a 50% resin content.
Aniline-formaldehyde resin has very poor flow properties and may
be moulded only with difficulty, and mouldings are confined to simple
shapes. The resin is essentially thermoplastic and does not cross-link with
the evolution of volatiles during pressing. Long pressing times, about 90
minutes for a 1/2 in thick sheet, are required to achieve a suitable product.
Laminated sheets may be made by plying up the impregnated paper
and pressing at 20MPa moulding pressure and 160-170°C for 150 minutes,
followed by 75 minutes cooling in a typical process. A few shaped
mouldings may also be made from impregnated paper, by moulding at
higher moulding pressures.
As with the other aminoplastics, the chemistry of resin formation is
incompletely understood. It is, however, believed that under acid
conditions at aniline-formaldehyde ratios of about 1:1.2, which are similar
to those used in practice, the reaction proceeds via p-aminobenzyl alcohol
with subsequent condensation between amino and hydroxyl groups:
It is further believed that the excess formaldehyde then reacts at
the ortho-position to give a l ightly c ross-linked polymer with very limited
thermoplasticity:
Such condensation reactions occur on mixing the two components.
The resultant comparative intractability of the material is one of the main
reasons for its industrial eclipse.
1.3.5 Resins Containing Thiourea
Thiourea may be produced either by fusion of ammonium
thiocyanate or by the interaction of hydrogen sulphide and cyanamide.
83
NH
4
SCN
CS(NH
2)2
NH2CN + H
2
S
CS(NH
2
)
2
The first process is an equilibrium reaction which yields only a
+ CH
2
O
+ H
2
O
NHCH
2
OH
+ HOCH
2
NH
NHCH
2
NH
HOCH2NH+
NHCH
2
OH
CS
NHCH2NH
CS
+ H2O
25% conversion of thiourea after about 4 hours at 140-145°C. Prolonged or
excessive heating will cause decomposition of the thiourea whilst pressure
changes and catalysts have no effect on the equilibrium. Pure thiourea is a
crystalline compound melting at 181-182°C and is soluble in water.
Thiourea will react with neutralized formalin at 20-30°C to form
methylol derivatives which are slowly deposited from solution. Heating of
methylol thiourea aqueous solutions at about 60°C will cause the formation
of resins, the reaction being accelerated by acidic conditions. As the resin
average molecular weight increases with further reaction the resin becomes
hydrophobic and separates from the aqueous phase on cooling. Further
reaction leads to separation at reaction temperatures, in contrast to ureaformaldehyde resins, which can form homogeneous transparent gels in
aqueous dispersion.
Polymer formation is apparently due to hydroxymethyl-methyl and
hydroxy-methyl-amino reaction:
In comparison with urea-based resins, thiourea resins are slower
curing and the products are somewhat more brittle. They are more waterrepellent the U-F resins.
At one time thiourea-urea-formaldehyde resins were of importance
for moulding powders and laminating resins because of their improved
water resistance. They have now been almost completely superseded by
melamine-formaldehyde resins with their superior water resistance. It is,
however, understood that a small amount of thiourea-containing resin is
still used in the manufacture of decorative laminates.
1.4 Heterochain Polyesters
Polyesters are encountered in many forms. They are important as
laminating resins, moulding compositions, fibres, films, surface coating
resins, rubbers and plasticizers. The common factor in these widely
different materials is that they all contain a number of ester linkages in the
84
main chain. (There are also a number of polymers such as poly(vinyl
HORCOOH + HORCOOH etc
ORCOORCOO
+ H
2
OHOROH + HOOCR
1
COOH + HOROH
OROOCR
1
COORO
R1OOCRCOOR
1
+ HOR
2
OH
OOCRCOOR
2
OO
+ R1OH
R O
C
O
RCOO
ClOCROCl + HOR
1
OH
OCRCOOR
1
O
+ HCl
acetate) which contain a number of ester groups in side chains but these are
not generally considered within the term polyester resins.)
These polymers may be produced by a variety of techniques, of
which the following are technically important:
(1) Self-condensation of ω-hydroxy acids, commercially the least
important route:
(2) Condensation of polyhydroxy compounds with polybasic acids,
e.g. a glycol with a dicarboxylic acid:
(4) Ring opening of a lactone, e.g. of ε-caprolactone with
dihydroxy or trihydroxy initiators:
(3) Ester exchange:
(5) Alcoholysis of the acid chloride of a dicarboxylic acid with a
polyhydroxy alcohol:
Credit for the preparation of the first polyester resin is given
variously to Berzelius1 in 1847 and to Gay-Lussac and Pelouze in 1883.
Their first use came about in the early years of this century for surface
coatings where they are well known as alkyd resins, the word alkyd being
derived somewhat freely from alcohol and acid. Of particular importance
in coatings are the glyptals, glycerol-phthalic anhydride condensates.
Although these materials were also used at one time for moulding materials
they were very slow curing even at 200°C and are now obsolete and quite
different from present day alkyd moulding powders.
Linear polyesters were studied by Carothers during his classical
researches into the development of the nylons but it was left to Whinfield
and Dickson to discover poly(ethylene terephthalate), now of great
importance in the manufacture of fibres (e.g. Terylene, Dacron) and films
(e.g. Melinex, Mylar). The fibres were first announced in 1941.
At about the same time, an allyl resin known as CR39 was
introduced in the United States as a low-pressure laminating resin. This
was followed in about 1946 with the introduction of unsaturated polyester
85
laminating resins which are today of great importance in the manufacture
of glass-reinforced plastics. Alkyd moulding powders were introduced in
1948 and have since found specialized appl icatio ns as elec tric al insu la tors.
With the expiry of the basic ICI patents on poly(ethylene
terephthalate) there was considerable development in terephthalate
polymers in the early 1970s. More than a dozen companies introduced
poly(butylene terephthalate) as an engineering plastics material whilst a
polyether-ester thermoplastic rubber was introduced by Du Pont as Hytrel.
Poly(ethylene terephthalate) was also the basis of the glass-filled
engineering polymer (Rynite) introduced by Du Pont in the late 1970s.
Towards the end of the 1970s poly (ethylene terephthalate) was used for
the manufacture of biaxially oriented bottles for beer, colas and other
carbonated drinks, and this application has since become of major
importance. Similar processes are now used for making wide-neck jars.
Highly aromatic thermoplastic polyesters first became available in
the 1960s but the original materials were somewhat difficult to process.
These were followed in the 1970s by somewhat more processable
materials, commonly referred to as polyarylates. More recently there has
been considerable activity in liquid crystal polyesters, which are in interest
as self-reinforcing heat-resisting engineering thermoplastics.
Such is the diversity of polyester materials that it has to be stressed
that their common feature is only the ester (–COO–) link and that this often
only comprises a small part of the molecule. Nevertheless it may influence
the properties of the polymer in the following ways:
1. It is, chemically, a point of weakness, being susceptible to
hydrolysis, ammonolysis and ester interchange, the first two reactions
leading to chain scission. In some cases the reactivity is influenced by the
nature of the adjacent groupings.
2. As a polar group it can adversely affect high-frequency
electrical insulation properties. Its influence is generally lower below Tg
unless the portion of the polymer containing the ester group has some
mobility below the main Tg.
3. The polar ester group may act as a proton acceptor, allowing
interactions with other groupings either of an inter- or an intramolecular
nature.
4. The ester link appears to enhance chain flexibility of an
otherwise polymethylenic chain. At the same time it generally increases
interchain attraction and in terms of the effects on melting points and
rigidity the effects appear largely self-cancelling.
86
1.4.1 Unsaturated Polyester Laminating Resins
The polyester laminating resins are viscous, generally pale yellow
coloured materials of a low degree of polymerization (~8-10), i.e.
molecular weight of about 2000. They are produced by condensing a glycol
with both an unsaturated and a saturated dicarboxylic acid. The unsaturated
acid provides a site for subsequent cross-linking whilst provision of a
saturated acid reduces the number of sites for cross-linking and hence
reduces the cross-link density and brittleness of the end-product. In practice
the polyester resin, which may vary from a very highly viscous liquid to a
brittle solid depending on composition, is mixed with a reactive diluent
such as styrene. This eases working, often reduces the cost and enhances
reactivity of the polyester. Before applying the resin to the reinforcement a
curing system is blended into the resin. This may be so varied that curing
times may range from a few minutes to several hours whilst the cure may
be arranged to proceed either at ambient or elevated temperatures. In the
case of cold-curing systems it is obviously necessary to apply the resin to
the reinforcement as soon as possible after the catalyst system has been
added and before gelation and cure occur. The usual reinforcement is glass
fibre, as a preform, cloth, mat or rovings but sisal or more conventional
fabrics may be used.
Since cross-linking occurs via an addition mechanism across the
double bonds in the polyesters and the reactive diluent there are no
volatiles given off during cure (c.f. phenolic and amino-resins) and it is
thus possible to cure without pressure. Since room temperature cures are
also possible the resins are most useful in the manufacture of large
structures such as boats and car bodies.
Small quantities of higher molecular weight resin in powder form
are also manufactured. They are used in solution or emulsion form as
binders for glass-fibre preforms and also for the manufacture of
preimpregnated cloths.
Selection of raw materials
1,2-Propylene glycol is probably the most important glycol used in
the manufacture of the laminating resins. It gives resins which are less
crystalline and more compatible with styrene than those obtained using
ethylene glycol. Propylene glycol is produced from propylene via
propylene oxide. The use of glycols higher in the homologous series gives
products which are more flexible and have greater water resistance. They
do not appear to be used on a large scale commercially.
87
Products such as diethylene glycol and triethylene glycol, obtained
CH
3
CH
CH
2
OH
OH
1,2-Propylene Glycol
HO
CH
2
CH
2
O CH
2
CH
2
OH
Diethylene Glycol
C
C
H
HOOC
H
COOH
H
COOH
HOOC
H
C
C
HC
C
O
C
CH
O
O
Maleic Acid Fumaric Acid
Maleic Anhydride
by side reactions in the preparation of ethylene glycol, are sometimes used
but they give products with greater water absorption and inferior electrical
properties:
Most conventional general purpose resins employ either maleic
acid (usually as the anhydride) or its trans-isomer fumaric acid (which does
not form an anhydride) as the unsaturated acid:
Maleic anhydride is commonly prepared by passing a mixture of
benzene vapour and air over a catalyst (e.g. a vanadium derivative) at
elevated temperatures (e.g. 450°C). It is a crystalline solid melting at
52.6°C (the acid melts at 130°C).
Fumaric acid may be prepared by heating maleic acid, with or
without catalysts. It is also obtained as by-product in the manufacture of
phthalic anhydride from naphthalene. The acid is a solid melting at 284°C.
Fumaric acid is sometimes preferred to maleic anhydride as it is less
corrosive, it tends to give lighter coloured products and the resins have
slightly greater heat resistance.
Saturated acids
The prime function of the saturated acid is to space out the double
bonds and thus reduce the density of cross-linking. Phthalic anhydride is
most commonly used for this purpose because it provides an inflexible link
and maintains the rigidity in the cured resin. It has been used in increasing
proportions during the past decade since its low price enables cheaper
resins to be made. The most detrimental effect of this is to reduce the heat
resistance of the laminates but this is frequently unimportant. It is usually
produced by catalytic oxidation of o-xylene but sometimes naphthalene and
is a crystalline solid melting at 131°C.
88
C
O
C
O
O
COOH
COOH
HOOC(CH
2)4
COOH
Phthalic anhydride Isophthalic Acid
Adipic Acid
Isophthalic acid (m.p. 347°C), made by oxidation of m-xylene, has
also been introduced for resins. The resins have higher heat distortion
temperatures and flexural moduli and better craze resistance. They are also
useful in the preparation of resilient gel coats.
Systems based on isophthalic acid often show better water and
alkali resistance than those based on phthalic anhydride. This is not
thought to be due to inherent differences between the phthalic and
isophthalic structures but is ascribed to the fact that isophthalate resins
have generally considerably higher viscosities which enable them to be
diluted with greater amounts of styrene. It is the additional proportion of
styrene which gives the improved water and alkali resistance.
Where a flexible resin is required adipic and, rarely, sebacic acids
are used. Whereas the phthalic acids give a rigid link these materials give
highly flexible linkages and hence flexibility in the cured resin. Flexible
resins are of value in gel coats.
Diluents
Because of its low price, compatibility, low viscosity and ease of
use styrene is the preferred reactive diluent in general purpose resins.
Methyl methacrylate is sometimes used, but as it does not copolymerize
alone with most unsaturated polyesters, usually in conjunction with
styrene in resins for translucent sheeting. Vinyl toluene and diallyl
phthalate are also occasionally employed.
Production of resins
Polyester laminating resins are produced by heating the component
acids and glycols at 150-200°C for several hours, e.g. 12 hours. In order to
obtain a good colour and to prevent premature gelation the reaction is
carried out under an inert blanket of carbon dioxide or nitrogen. The
reaction mixture is agitated to facilitate reaction and to prevent local
overheating. A typical charge for a general purpose resin would be:
Propylene glycol 146 parts
Maleic anhydride 114 parts
89
Phthalic anhydride 86 parts
The molar ratio of these three ingredients in the order above is
1.1:0.67:0.33. The slight excess of glycol is primarily to allow for
evaporation losses. Xylene is often used to facilitate the removal of water
of condensation by means of azeotropic distillation. The reaction is
followed by measuring the acid number of small samples periodically
removed from the reactor. (The acid number is the number of milligrams of
potassium hydroxide equivalent to the acidity present in one gram of resin.)
Where there are equimolecular proportions of glycol and acid the number
average molecular weight is given by 56000/acid number. Since there is
some deviation from equimolecular equivalence in practice, care should be
taken in using this reationship. Reaction is usually stopped when the acid
number is between 25 and 50, the heaters are switched off and any xylene
presents is allowed to boil off into a receiver.
When the resin temperature drops below the boiling point of the
reactive diluent (usually styrene) the resin is pumped into a blending tank
containing suitability inhibited diluent. It is common practice to employ a
mixture of inhibitors in order to obtain a balance of properties in respect of
colour, storage stability and gelation rate of catalyzed resin. A typical
system based on the above polyester fomulation would be:
Styrene 148 parts
Benzyltrimethylammonium chloride 0.38 parts
Hydroquinone 0.05 parts
Quinone 0.005 parts
The blend is allowed to cool further and the resin is transferred
into drums for shipping and storage.
Quality control tests on the resins most commonly employed are
for specific gravity, viscosity, colour, clarity and gel time under standard
conditions, including fixed amount of curing system.
Curing systems
The cross-linking reaction is carried out after the resin has been
applied to the glass fibre. In practice the curing is carried out either at
elevated temperatures of about 100°C where press mouldings are being
produced, or at room temperature in the case of large hand lay-up
structures.
Benzoyl peroxide is most commonly used for elevated temperature
curing. The peroxide is generally supplied as a paste (~50%) in a liquid
such as dimethyl phthalate to reduce explosion hazards and to facilitate
90
mixing. The curing cycle in pressure moulding processes is normally less
than five minutes.
In the presence of certain aromatic tertiary amines such as
dimethylaniline, benzoyl peroxide will bring about the room temperature
cure of general purpose polyester resins.
More frequently either methyl ethyl ketone peroxide or
cyclohexanone peroxide is used for room temperature curing in
conjunction with a cobalt compound such as a naphthenate, octoate or
other organic solvent-soluble soap. The peroxides (strictly speaking
polymerization initiators) are referred to as “catalysts” and the cobalt
compound as an “accelerator”. Other curing systems have been devised but
are seldom used.
Structure and properties
The cured resins, being cross-linked, are rigid and do not flow on
heating. The styrene, phthalic anhydride, maleic anydride and propylene
glycol residues are predominantly hydrocarbon but are interspersed with a
number of ester groups. These latter groups provide a site for hydrolytic
degradation, particularly in alkaline environments. The polar nature of the
ester group leads to the resin having a higher power factor and dielectric
constant than the hydrocarbon polymers and this limits their use as highfrequency electrical insulators.
Many mechanical properties are dependent on the density of crosslinks and on the rigidity of the molecules between cross-links. It has
already been shown that cross-link intensity may be controlled by varying
the ratio of unsaturated to saturated acids whereas rigidity is to a large
extent determined by the structure of the saturated acid employed.
Polyester-glass fibre laminates
Glass fibres are the preferred form of reinforcement for polyester
resins since they provide the strongest laminates. Fabrics from other fibres
may, however, be used and can in some instances provide adequate
reinforcement at lower cost. Glass fibres are available in a number of
forms, of which the following are the most important:
(1) Glass cloth. A range of cloths is available and the finest of
these are used in order to obtain the best mechanical properties. They are,
however, expensive in use and they are used only in certain specialized
applications such as in the aircraft industry and for decorative purposes.
(2) Chopped strand mat. This consists of chopped strands (bundles