Study on direct-chain diacid modified phenolic
resin for Al-alloy casting
*Yundong JI
1, Jirong LUO
1, Moyin LI
2(1. School of Materials, Huangzhong University of Science and Technology, Wuhan 430074, Hubei, P. R. China. 2. Wuhan Life Chemistry and Industry Co., Wuhan 430011, Hubei, P. R. China.)
Abstract: Resin coated sand (RCS) with phenolic resin matrix can hardly be collapsed when it is used in Al-alloy casting. Adding collapsing agent and reducing the concentration of resin are solutions adopted by workers, but these methods tend to reduce the initial strength of RCS. Synthesis of modified phenolic resin with direct-chain diacid DAn (/JS=6, where n means carbon amount) was studied here. The effects of the addition of modifying agent on molecular weight, gel time and softening point were investigated. Optimal addition of DAn (10% phenol) was obtained by testing the initial and retained flexural strengths of the modified resin. FT-IR spectra showed that carbonyl shifts to higher wave number. With the use of TG, SEM and strength loss curves, the relation between initial and retained strengths was analysed. Tests on the heated deformation curve, before and after resin modification, show that PF-DA10 has the characteristic of higher initial and retained strengths together.
Keywords: direct-chain diacid; PF resin; modify; Al-alloy casting; RCS
CLC number: TG221 Document: A Article ID: 1672-6421(2005)01-0028-06
1. Introduction
The data shows that the oil consumption of a car is decreased by 6 %~8 % when its weight is reduced by
10%. So it is very clear that the cars' producers tend to reduce the weight of a car. Now the output of domestic cars is 20 million annually, the weight of light alloy casting per car is approximately 60 kg, while it has exceeded 100 kg in the developed countries. The total output of casting in China reaches 16.2 million tons, which is the largest in the world, being 1.3 times the output of the USA and 3 times that of Japan. But the output of light alloy casting is only 0.97 million tons in China, compared to 1.8 million tons in the USA and 1.2 million tons in Japan. It shows that the structure of domestic casting is not reasonable; and that it is imperative to develop light alloy casting.
Resin-coated sand (RCS) with phenolic resin matrix has been widely used in the production of sand core for Al-alloy casting due to its characteristics of high strength, casting precision and high-efficiency production. However, due to lower pouring temperature with Al-alloy casting, common phenolic resin is insufficiently broken down, and the sand core hardly collapses [1-3]. The works usually adopt the methods of adding collapsing agent and
reducing addition of resin so as to reduce retained strength of moulding sand, which is likely to reduce the initial strength, leading to some new problems such as waster ratio going up, etc [4-7].
RCS is a special composite with resin matrix, and resin is a precursor of carbon. Before pouring, initial strength has a bearing on toughness, quantity and interface property of the resin matrix. After pouring, retained strength is concerned closely with the conformation and distribution of pyrolytic carbon. Adding collapsing agent and reducing the addition of resin will have an effect on both initial and retained strengths. Therefore reduced retained strength often follows the reduction of initial strength [1,8-11]. Increasing the toughness of resin is an approach to improving initial strength. If the distribution of carbon can be reduced at the same time, it is possible to obtain RCS with high strength and easy collapsibility.
This paper mainly studies the synthesis of modified phenolic resin with direct-chain diacid DAn (n 3= 6, n-means carbon amount). Using FT-IR, the change of resin structure before and after being modified was studied, and the effects on molecular weight, gel time and softening point of the addition of modifying agent were investigated. The initial and retained flexural strengths of modified resin were tested, so as to ascertain the optimal addition of modifying agent. Making use of TG, SEM and strength loss curve, the relationship between initial and retained strengths was analysed. Finally, using our own design of instrument for a moulding sand heat *Yundong JI: Associate Prof., research field: composite materials
with resin matrix, and special macromolecular binder, foundrytesting instrument.
E-mail: feederji@sina.com
Vol. 2 No. 1 Direct-chain diacid modified phenolic resin for Al-alloy casting
deformation experiment, the process of RCS heat
deformation before and after resin modification were
studied. This data supplied the experimental foundation
for the practicality of RCS.
2. Experiment
2.1 The synthesis of modified resin
The raw materials used in the research contain mainly
phenol and formaldehyde (mol. ratio of the two is 1:0.84),
hydrochloric acid and direct-chain diacid DAn (0%~30 %
/phenol). All reagents were obtained from Tientsin
Reagents Factory, with no refinement. The flow process
is as shown in Fig. 1.
marked δr(T), where T is the holding temperature.
2.3 Thermo-gravimetric analysis and observation on
fracture morphology of RCS
TGA (DT-30B) was used to observe the thermo-gravimetric curves of the seven specimens under a
nitrogen atmosphere. The scan rate was 10 °C/min.
The retained strength of RCS at different temperatures
was tested. The section of the specimen after fracture was
kept. The fracture surface of RCS was coated with a thin
layer of a gold-palladium alloy to image its morphology
on the JSM-35C SEM.
2.4 Test on RCS heated deformation curve
The deformation curve of a heated sand specimen was
tested with an instrument of our own design. Fig.2 shows the working principle of the equipment [12]. The shape of
the specimen is rectangular with dimensions of 22.4 mm× 11.2 mm × 200 mm. The equipment simulates the pouring condition of the moulding sand. The result of the experiment is important in checking whether the moulding sand can be used in practical applications.
Fig.1 The synthesis process flow
By changing the mixture radio (0%~30 %) of DAn and phenol, seven groups of resin specimens were obtained, named respectively as PF, PF-DA5, PF-DA10, PF-DA15, PF-DA20, PF-DA25, and PF-DA30 (the number indicates the DAn addition). The softening point and gel time of the specimens were tested. FT-IR spectra was used to study the reaction of the modified PF resin at 0.5 cm-1 resolution on the FQUINOX55 spectrometer. Number average molecular weight was measured on the 1100 series GPC meter.
2.2 Initial flexural strength and retained flexural
strength of RCS
To mix the RCS: to 1 kg of rough sand, add 2.0% of resin, 1.5% of water, and 0.1% of stearic acid calcium. Using the above resin specimens, seven specimens of RCS were obtained, named separately as SO, S5, S10, S15, S20, S25, and S30.
The preparation and test of initial flexural strength were based on JB/T8583-1997. The initial flexural strength specimen was marked as δi. The preparation of a retained flexural strength specimen was as follows: after the
specimen was used for initial flexural strength, it was
wrapped tightly in tinfoil, put into the oven for 30 minutes
at a certain temperature, then taken out for cooling to room temperature. According to the JB/T 8583-1997
method, it was tested for retained flexural strength, and
Fig.2 The sketch of heated deformation sample
3. Results and analysis
3.1 Synthesis of modified phenolic resin
Synthesis of the phenolic resin contains a two-step reaction of addition and then condensation, as shown in
Fig.3(a) and (b). Phenol and formaldehyde were reacted
firstly, forming hydroxymethyl substituted phenol, then
the condensation reaction with phenol, forming phenol
condensation with methylene bridge juncture [13-15] . Fig.3
(c) shows the esterification between diacid DAn and
active hydroxymethyl. Flexible units were inserted in the
rigid fundamental chain of phenolic resin so the toughness
of the resin was improved.)
The reaction of diacid DAn modified PF was investi-gated with various addition of DAn using FT-IR. In Fig.4,
the broad peak around 3 200~3 700 cm-1
is characteristic
of the -OH stretchings of a phenolic ring, the methylol
group of phenolic resin, and diacid. The small peaks
at 2 800~3 060 cm-1
stretchings of the phenolic ring, the methylene (-CH2-) and dimethylene ether (-CH2OCH2) bridges of phenolic resin, and the methylene (-CH2-) unit of diacid DAn. A signal at 1 610 cm-1 is characteristic of the elong- ation of the aromatic ethylene bond (C=C) of the phenolic ring. The signals at 753~794 cm-1, 820~ 855 cm-1, and 912~ 917 cm-1 can be assigned to ortho and para substitution and ring deformation of a phenolic ring, respectively. The peak around 1 200 cm-1 is associated with -CO stretchings of a phenolic ring, dimethylene ether,and diacid. The band at 1 500 cm-1 is assigned to phenolic ring substituted in ortho or para positions. A peak at 1 700 cm-1 corresponds to the -C=O stretching of diacid DAn. From PF-DA15, the intensity of the -OH peak around
3200~3350 cm-1 increases, maybe free DAn appears. The carbonyl signal shifts to a higher wave number with the addition of DAn. It indicates that the carbonyl acid is transformed into ester linkage to react with actively hydroxymethyl [16]. The intensity of the peak around 912 ~ 917 cm-1 increases with the addition of DAn. It shows the phenolic ring deformation is easier due to the flexible units in the fundamental chain of PF.
The gel time and the softening point have important impact on resin application. Fig.5 and Fig.6 show the results of testing gel time and softening point. We tested number average molecular weight of the specimen, as shown in Table 1.
Fig.5 The effect of DAn addition on gel time Fig.3 The reactions of DAn modified PF
Fig.4 FT-IR spectra the modified PF with various addition of DAn
Fig.6 Effect of DAn addition on softening point
Vol. 2 No. 1 Direct-chain diacid modified phenolic resin for Al-alloy casting
With the increase of DAn, the gel time of resin first
decreases and then increases, the shortest gel time, for
PF-DA10, is 22 seconds. When the concentration of DAn is under 10%, all the modifier is for reaction, and the
molecular weight increases quickly, so the gel time is
shortened. For a concentration of DAn of more than 10%,
due to the appearance of free diacid, the addition of diacid
in reaction is a little increasable; the increase in relevant
average molecular weight becomes slower. Furthermore,
the flexibility of resin increases, and gel time is prolonged
due to the reduction of phenol proportion. With
increasing DAn concentration, the softening point
decreases steadily, which is likely to have a bearing on the
increase of flexible groups and the weakness of the forces
acting between the molecules.
3.2 Relation between initial strength δi and retained
strength δr(T) of modified phenolic resin
As shown in Fig. 7, when the concentration of DAn is
less than 10%, δi goes up with the increase of DAn, the greatest strength, of S10, being 6.3 MPa, which is 20%
more than S0. The improvement of strength is likely to
result from the improvement of the toughness of modified
phenolic resin. Once the concentration of DAn is more
than 10%, δi begins to fall, which is likely to result from
free diacid. Fig.8 shows the relationship between δr(500)
and DAn concentrations.δr(500) changes according to the
increase of DAn concentration, δr(500) reaches its lowest
value, of 0.3 MPa, for S10; then the change becomes much less. The result shows that the addition of DAn has
contrary impact on δi and δr (500). The result is in complete contrast to the traditional viewpoint of
consistent linearity of initial and retained strengths[1]
.
Fig.7 The effect of DAn addition on δi Fig.8 The effect of DAn addition on δr
In order to explain the result, TGA of the RCS speci-men was investigated. Fig.9 shows that once S10 reaches
the temperature of about 350 °C , speedy weight loss
occurs. The thermo-gravimetric curves of S15, S20, S25
and S30 are similar. S5 begins weight loss at a
temperature of about 450 °C, while S0 begins obvious
weight loss at a temperature of 500 °C. The result shows
that thermal stability of modified phenolic resin becomes
poor. There are two underlying reasons. Firstly, the
insertion of flexible units in the fundamental chain of the
phenolic resin, makes it easier to decompose the chain
than the phenolic one. Secondly, the bond energy of the
existing C-O bond of the phenolic base in the fundamental chain is lower than that of the C-C bond, so it can be more
easily disconnected. The heat stability of S15, S20, S25
and S30 is better than that of S10, which is likely to be
because their molecular weight is bigger than S10. If we compare Fig.8 and Fig.9, the results are consistent.
Maybe it is because the thermal decomposition of resin
determines in quantity and distribution of pyrolytic
carbon, so the retained strength has a close relationship
with the thermo-gravimetric curve.
In order to further explain the relationship between
initial strength and retained strength, we chose S10 and S0, studied contrastively retained flexural strengths under
different holding temperatures, and observed the fracture
morphology of specimens with SEM. Fig.10 shows the
curves for retained strengths from room temperature to
600 °C, and we call them strength loss curves. Between
25 and 350 °C the flexural strength declines linearly, with
S10 declining faster than S0. Between 350 and 400 °C a
'strength platform' at 2.1 MPa occurred in the curve for
S10, and between 400 and 450 °C a 'strength platform' at
2.5 MPa occurred in the curve for S0. At higher
temperatures, the strength declines swiftly. At 500 °C the
strength of S0 is 1.2 MPa while that for S10 is only 0.3
MPa. This is consistent with the 'strength platform' for S0
appearing at a higher temperature than that for S10. The
'strength platform' is probably related to the conformation
and distribution of pyrolytic carbon [12,17]
Fig. 11 (a) is the fracture morphology of S10 at room temperature; it is shown that the resin was lacerated. The surface of the RCS is not smooth. Fig. 11 (b) and (c) show specimen fracture goes to lubricity at under 350 °C ,
without lacerated trace, just brittle rupture, the surface of the sand bead is lubricous. The fracture and adhering bridge morphology at 450 °C is not very different from
that at 350 °C. This characteristic corresponds to the position of the platform in the strength loss curve. At 550
°C, the resin neck is quite thin, approaching dissolved; the coarse surface of raw sand is obvious, the camber of adhering bridge has disappeared, becoming rod-juncture, resin neck is strictly ablated, retained strength corresponding to it appearing quite low.
Fig.9 TG curve of RCS 10 °C/min
Fig.10 The relationship between stress and baking temperature
Fig. 11 Fracture morphology of RCS
3.3 Test of heated deformation curve and evaluation of practicability
Due to test property of moulding sand under the condition of simulated casting, the heated deformation curve is important to evaluate whether matrix material of moulding sand can be applied. The research tested heated deformation of PF and PF-DA10 taken as S0 and S10 of matrix material. As shown in Fig. 12, we changed the addition of resin in order to find out adjustment range of PF-DA10 addition. Mixed RCS with 1.0% is marked as
S10-1 (δi = 4.4 Mpa), and RCS with 2.0% as S10-2.
Vol. 2 No. 1 Direct-chain diacid modified phenolic resin for Al-alloy casting
Reducing use level of PF-DA10, heated deformation curve of S10 has no obvious change, externalising the characteristics of PF-DA10 high collapsibility and wide adjustment range of resin addition.
Fig.12 Heat bending curve of S10 and S0
4. Conclusions
(1) The study of the structure of diacid DAn (n^6)
modified phenolic resin by FT-IR. The 1700 cm-1
carbonyl signal tends to a higher wave number with the increase of DAn, which shows the occurrence of esterification reaction. The intensity of the peak at 912 cm-1
, assigned as phenol ring, increases with the addition of DAn, showing that deformation increases gradually due to the insertion of flexible segments in the phenolic resin rigidity fundamental chain. From the jump of PF-DA15, the -OH peak intensity of modified resin increases, which shows that 10%~15 % of DAn is a suitable amount. Result of the strength test also supports this conclusion.
(2) With the increase of DAn, the gel time of the resin decreases at first and later increases, the shortest gel time, for PF-DA10, is 22 seconds; the softening point decreases; the relevant average molecular weight increases at first, and then the rate of increase becomes slower. This shows that the DAn improves the degree of the cross linking of the resin, at one time flexible segment in the phenolic resin decrease cross speed of resin and weaken the forces acting between the molecules.
(3) Comparing mixed RCS of the studied modified resin with non-modified resin, the modification improves initial flexural strength δi, but reduces retained flexural strength
δr(T). Making use of TG, SEM and strength loss curve,
the relationship between δi and δr(T) was analysed. δr(T)
mainly bears on the thermo-gravimetric curve, but has no direct relationship with origination of strength loss curve.
By means of inserting flexible units in the rigidity fundamental chain of phenolic resin, resin toughness is improved and the thermo-gravimetric curve is changed, so higher initial strength and lower retained strength are obtained.
(4) PF-DA10 was examined under simulated casting conditions by determining its heated deformation curve. The result shows that RCS mixed with different amount PF-DA10 can satisfy for the requirements of simulated casting conditions, and externalise the quality of high collapsibility. Resin with low addition has a heated deformation curve similar to that for the resin with high addition, which shows that PF-DA10 addition has no impact on its collapsibility.
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