Journal of

ELECTROSTATICS ELSEVIER Journal of Electrostatics 44 (1998) 53-60

EFFECT OF PULVERIZATION ON CHARGE TRAPPINGPROPERTIES OF POLYMETHYL,METHACRYLATE.

Yuki YAMADA and Kazuo IKEZAKI

Department of Appliedphysics and Physico-lnformatics, Faculty of Science and Technology,

KEIO Universiry.

3-14-l. Hiyoshi, Kohoku-ku, Yokohama, JAPAN.

Thermally stimulated current (TSC) spectra of powder-formed polymethylmethacrylate (PMMA) of different particle sizes were observed in a temperature range between room temperature and 160 ‘C. From comparison of these TSC spectra with that of melt-cast PMMA samples, it was found that pulverization produces two kinds of new charge trapping sites which were shallower and deeper than the inherent charge traps in PMMA. These newly produced charge trapping sites were annealed out by appropriate heat treatments of sample powders. The depth of the charge traps inherent in PMMA themselves was once reduced slightly by pulverization but recovered toward the original depth by a thermal effect of further pulverization.

Keywords: polymethylmethacrylate, powder, thermally stimulated current, charge trap.

1. Introduction

In the industrial fields of copying machine and electrostatic powder coating, pulverization of polymer is an important unit process for producing a toner and a powder paint. However, the effect of pulverization on the charge trapping properties of polymers has not been fully examined yet. In these fields, charging characteristics of toners or powder paints

have been exclusively evaluated by the blow-off method [l] and the inclined plate method [2]. However, these methods can be used not for a given material itself but for a pair of substances such as a toner and a carrier. In addition to this, these methods can only evaluate compound

factors of generation and retention of charges as a whole. For better understanding of charging

characteristics, these two factors are expected to be separately examined. For evaluating the charge retention factor, thermally stimulated current (TSC) spectroscopy

[3] is the best technique. As far as we know, however, any TSC study of powder-formed polymers has not been published yet except our few studies [4,5].

In this paper, we report the effect of pulverization of polyrnethylmethacrylate (PMMA) on its charge retention factor with the TSC technique.

2. Experimental

We used atactic-PMMA beads of about 200 urn diameter with the molecular weight of 35000 and the glass-rubber transition temperature T, of 105 C, which was a product of Scientific Polymer Products Inc., as a starting material. With plenty of liquid nitrogen the

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54 I! Yamada, K. Ikezaki/Journal of Electrostatics 44 (1998) 53-60

PMMA beads were cryogenically pulverized for 20 set at a time with a cutting mill which had been fully cooled beforehand with liquid nitrogen. For getting smaller size powders, the

cryogenical pulverization process was repeated. Obtained PMMA powders were classified with multistage sonic sieves into four classes depending on their particle size: 10-20, 20-38, 38-75 and 75-180 pm.

For TSC measurements, these classified powders of 116 mg were put in an Al sample

folder in the shape of a shallow well of 0.5 mm depth and 20 mm diameter and pressed at 55 kg/cm2 for 2 min. The thickness of the obtained powder compaction was controlled with a spacer. The powder-formed samples thus prepared were circular disk shape compactions of

about 0.5 mm thick and of 20 mm diameter. We also used the melt-cast PMMA thin films as standard samples. The melt-cast samples were prepared in the following way: similarly to the

powder-formed samples, the same amount of powders of 116 mg was put into the sample holder and heated at 170°C for one hour and then cooled slowly to room temperature.

For these powder-formed and the melt-cast PMMA samples, we negatively corona-charged them to various initial surface potentials V, and observed their TSC spectra from room temperature to 160°C at a constant heating rate of 3.5”CYmin. We also observed differential scanning calorimetry (DSC) thermograms in order to know pulverization effect on the

molecular aggregation state of PMMA.

3. Experimental results and discussion

3.1 TSC spectra of the melt-cast PMMA

TSC spectra of the melt-cast PMMA samples with different V s are shown in Fig. 1. TSC spectra of PMMA have been already reported by many authors to have two bands above

room temperature: these two bands appear around and above Ts [6,7]. In agreement with

the literature, two TSC bands, Rt and R2, can be seen for our melt-cast PMMA sample with

2-

TEMPERATURE (“c)

Fig. 1 TSC spectra of melt-cast Ph4MA samples initially corona-charged to (a) -387 V, (b) -529 V,

(c) -761 V and (d) -919 V.

E Yamada. K. Ikezaki/Journal of Electrostatics 44 (1998) 53-60 55

low vs. As shown in the TSC spectrum of -387 V sample in Fig.1, a small band Rt and a large band R2 appeared at 10X, T, of the starting PMMA material, and 133°C respectively. For high V,, however, this small TSC band Rr was mashed behind R2. Fig.1

also shows that the temperature of the R2 maximum is independent of V,. This field independent nature of the peak temperature for the R2 band means that the charge transport process is not the rate determining process for the observed TSC of R2 [5].

3.2 TSC spectra of the powder-formed PMMA

TSC spectra of the powder-formed PMMA samples were found generally to have four

bands, Bo, BI, B2 and B3, and they appear around 40, 70, 130 and 146”C, respectively. As an example, TSC spectra of PMMA samples of 75-180 pm particle size powders are presented in Fig.2. Unlike the TSC spectra [4] of the binder-resin for toner and those of the melt-cast samples with low V, in the present study, powder-formed PMMA samples do not

have any TSC band around 105’C even for the low surface potentials. As for the Ba band, the TSC polarity of which is inverse to that of the other three bands, it can be seen in Fig.2 that this band become prominent when the initial surface potential of the samples increases. In addition to this, the Bc band could not be observed in samples of small particle size powders. Therefore, it can hardly be seen for samples of powders of such a smaller particle size as lo-20 pm. Because the origin of the band is not clear at present, no further discussion about this band will be made in this paper.

Meanwhile, the Br band, which is small and appears around 70°C increases with decreasing particle size of the sample powders. As shown in Fig.2, the peak position of the B2 band is lower by only about 3’C than that of R2 of the melt-cast samples and it does not depend on V, at all. Therefore, the origin of this band is thought to be the same as that of the R2 band.

Although the B3 band has an apparent peak shift toward the low temperature side by about 1 “C or so at high V,, the peak position of this band is also considered to be essentially V,-

,,I,,#, ,,,I ,I 50 100 150

TEMPERATURE (“c)

Fig.2 TSC spectra of powder-formed PUMA samples initially corona-charged to (a) -395 V,

(b) -588 V, (c) -755 and (d) -973. Particle size: 75-180 pm

56 Y Yamada. K. Ikezaki/Journal of Electrostatics 44 (1998) 53-60

independent because the observed slight peak shift is due to overlapping between this band and the increasing tail of the high temperature side of the B2 band.

Fig.3 shows the particle size dependence of the TSC spectra for the powder-formed

PMMA samples. In Fig.3, the following are clearly shown: As the particle size of the sample powders decreases, (1) the intensity of the B 1 band increases, (2) the peak position of the B2 band shifts toward that of the R2 band of the melt-cast samples as mentioned before, (3) the B3 band grows and the peak position of this band shifts to the low temperature side

and (4) the B2 and the B3 bands eventually overlap each other to make a single band peaked at 14O’C when the particle size decreased to 1 O-20 urn.

Particle size (pm) a: 75-180

d

b: 38-75 c: 20-38

1 d: lo-20

II 11 ” ‘1 ” ” “‘I 50 100 150

TEMPERATURE (“c)

Fig.3 Particle size dependence of the TSC spectra of powder-formed Ph4MA samples

initially corona-charged to (a) -588 V, (b) -590 V, (c) -589 V and (d) -584 V.

3.3 Annealing effect of the powder-formed PMMA samples on their TSC spectra.

From the TSC results shown in the preceding sections, pulverization of PMMA seems to produce new charge trapping sites which correspond to B 1 and B 3, and also change the R2 traps intrinsic to PMMA to the shallower B2 traps.

Pulverization of polymers induces many mechanical effects on these polymers. These mechanical effects such as compression, elongation, friction, shearing and so on bring about scission of polymer chains resulting in the molecular weight reduction. Extraction and

disentanglement of molecular chains are also brought about by these mechanical effects during pulverization. These mechanically induced changes in the molecular chains result in unfavored molecular aggregate states far from the thermal equilibrium. Consequently, new

charge trapping sites may be produced in the non-equilibrium molecular aggregations during pulverization.

On the other hand, these mechanical effects are considered to induce thermal effects such as partial melting of the polymer and / or annealing the mechanically induced non-equilibrium molecular aggregation states toward thermal equilibrium ones.

Y. Yamada. K. Ikezaki/Journal of Electrostatics 44 (1998) 53-60 57

If the charge trapping sites newly produced by pulverization originate from thermal non- equilibrium molecular aggregations, then these charge trapping sites must be annealed out by appropriate thermal treatments. For this reason, we tried to anneal the samples at different

temperatures and then measured the TSC spectra of these annealed samples.

TEMPERATURE (“c)

Fig.4 TSC spectra of powder-formed PMMA samples (a) before and (b) after annealing at 9 1 ‘C for 15 min. These samples were corona-charged to about -800 V. Particle size: 20-38 pm

The result of 91 “C annealing of 20-38 urn powders is shown in Fig.4. As shown in Fig.4, only a 15 min heat treatment at 91’C made almost all of the Br band intensity vanish. In order to determin the change in the molecular aggregate state with the heat treatment, we observed DSC thermograms of 20-38 urn PMMA powders before and after the heat treatment. The observed thermograms are shown in Fig.5.

II # 40 60 80 loo 120 140 160

TEMPERATURE (“c)

Fig.5 DSC thermograms of powder-formed PMMA samples (a) before and (b) after annealing at 91 ‘C for 15 min. Particle sizez 20-38 pm

58 Y Yamada. K. IkezakiIJournal of Electrostatics 44 (1998) 53-60

Although the untreated powders have a broad endothermic band around 60°C in its DSC thermogram, this endothermic band decreased for the sample PMMA powders which were heat treated at 9 1 ‘C for 15 min. Comparing these thermograms with the TSC spectra shown in Fig.4, we concluded that the broad endothermic DSC band around 6O’C was responsible for the B1 band around 70°C in TSC spectra of the powder-formed PMMA samples.

Other heat treatment experiments were also made : TSCs were measured for 75-180 pm powder samples which were annealed at 130°C for 45 min or at 15o’C for 20 min. Observed

spectra for these samples are presented, respectively, in Fig.6 and in Fig.7 with the TSC spectrum of the untreated sample.

0

TEMPERATURE (“c)

Fig.6 TSC spectra of powder-formed PMMA samples (a) before and (b) after annealing at 130-C for 45 min. These samples were corona-charged to about -600 V. Particle size: 75-180 pm

0

TEMPERATURE (“c)

Fig.7 TSC spectra of powder-formed PMMA samples (a) before and (b) a&r annealing at 150 ‘C for 20 min. These samples were corona-charged to about -540 V. Particle size: 75-180 pm

Y. Yamada. K. Ikezaki/Journal of Electrostatics 44 (1998) 53-60 59

As shown in Fig.6, the peak position of the Bz band shifted to 133’C which corresponds to the peak position of the R2 band of the melt-cast samples. Moreover, the initial rising part of the TSC of the annealed sample decreased in comparison to that of the untreated sample. It is also shown in Fig.6 that the intensity of the B3 band decreased to a shoulder.

When the annealing temperature was raised to 15O”C, higher than the peak position of the BZ band, the Bz band still remained only with a slight peak shift to the high temperature side

and a weak band appeared at 106°C though the B3 band completely disappeared as shown in Fig.7. Moreover, the spectral shape of the TSC of this annealed sample almost coincides with that of the melt-cast samples as is seen from Fig. 1 and Fig.7. These facts mean that the weak band and the B2 band have the same origins as those of Rt and R2, respectively. The newly

produced charge trapping sites during pulverization are thermodynamically unstable, while the R1 and R2 bands are stable and inherent in PMMA.

Here, we recall that the B2 band once shifted to the lower temperature side by pulverization

returned toward the position of the RZ band inherent in PMMA as the particle size reduction proceeded by further pulverization. Therefore, as for the B2 band the extreme pulverization

seems to have the same effect on the sample powders as the heat treatment at higher temperatures than 91’C, because the heat treatment at 91°C had no effect on the peak position of the B2 band as shown in Fig.4. As for the Bt band, however, the heat treatment

above 91°C reduced the Br band as shown in Fig.4. The fact actually observed is that the B 1 band increased in its intensity and in the same time the peak position of the B2 band shifted to the higher temperature side as the particle size of the sample powders decreased with further pulverization. This is a contradiction. Namely, for smaller particle size powders, increase in the B 1 band and the observed peak shift of the B2 band contradict each other.

One of the possible explanations for this apparently contradictory fact is as follows: When a particle of PMMA is fractured into two parts by impact, the temperature of the part nearest to impact rises near or above the melting point and then falls again rapidly by liquid nitrogen. Therefore, this part of the particle undergoes a high temperature heat treatment and partial melting and / or annealing are brought about in this part. On the contrary, the temperature of the parts slightly farther away from impact does not seem to rise so much. As a result, molecular aggregate states far from thermal equilibrium induced by extraction and disentanglement of the molecular chain are frozen in as they are. These thermal non- equilibrium states are considered to be the origin of the charge traps responsible for the B 1 and

B3 bands. In a word, cryogenically pulverized PMMA particles have a two-phase structure, consisting of thermal equilibrium and non-equilibrium phases. Such a two phase structure of cryogenically pulverized particles has been also reported by Lovinger et al.[8] for crystalline polymers. Their two-phase model consists of an amorphous skin that surrounds a damaged

crystalline core. In our model, however, the two phases are considered to coexist in the skin of the particle because of the following reason: When a PMMA powder bed is corona-

charged, charges penetrate inside the bed [91, but they are considered to be trapped in the surface region of each PMMA particle. As a result, TSC spectra reflect the molecular region of the particles. aggregation states of the surface

4. Conclusion

From these experimental results mentioned above, the following are concluded:

60 E Yamada, K. Ikezaki/Journal of Electrostatics 44 (1998) 53-60

(1) PMMA material used in this study has inherent charge trapping sites responsible for theTSC band peaked at 133°C.

(2) The energy depth of these inherent traps changes depending on the extent of

pulverization. (3) Pulverization produces new charge trapping sites. These charge trapping sites are thermally unstable and extinguished by appropriate heat treatments. (4) The molecular aggregation state of PMMA particles cryogenically pulverization with liquid nitrogen are of macroscopic inhomogeneity.

Acknowledgment.

This work is partly supported by Hosokawa Powder Technology Foundation. The authors would like to express their gratitude for this financial support.

References

[l] D.K.Donald, J.appl.phys., 40 (1969) 3013.

[2] H.W.Gibson, J.M.Pochan and F.C.Bailey, AnaLChem. 51 (1979) 483.

[3] R.Chen and Y.Kirsh, Anu@s of&rmal& stimulatedprocesses (Pergamon Press, 1981) pp.60.

[4] T.Hori and KIkezaki, Proc.Znd lnt.Conf.on Image Science and Hardcopy ‘95 (Guilin, China, 1995),

pp.104.

[S] T.Hori and K.Ikezaki, J.Electrostat., 40 &41(1997) 3 13.

[6] J.Vandershueren and J.Gasiot, Thermally Stimuluted Relax&ion in Solids ed. by P.Braunlich ( Springer-

Verlag, 1979 ), pp.141.

[7] AChat%, D.Chatain, JDugas, C.Lacabanne and E.Vayssie, J.Macromol.Sci-Phys., B22 (19834) 633.

[8] A.J.Lovinger, L.A.Belflore and T.N. Bowmer, J.Polym.Sci.Poly.Phys.Edit., 23 (1985)1449.

[9] M.Takeuchi and H.Nagasaka, J.Electrostat., 13 (1982) 175.