Super Tough PVC Alloy With High Heat Distortion Temperature

GIA HUYNH-BA

E.I. DuPont de Nemours & Co. Marshall Laboratory, 3500 Grays Ferry Avenue

Philadelphia, Pennsylvania 19146

Poly(viny1 chloride) (WC) is an inexpensive commodity polymer with many good properties such as flame retardancy and rigidity. Unfortunately, its heat distortion temperature is around 75°C. To increase its toughness, plasticizers or elastomers are added with a loss in both heat distortion temperature (HDT) and flame retard- ancy. Low molecular weight imidized acrylic having a Tg < 130°C when added to WC shows a modest gain in HDT but at the expense of toughness. Inclusion of a rubber to improve the toughness of the acrylic modified composition lowers its HDT as well as its flame retardancy. The technology reported here details a method utilizing an “alloy” polymer added to WC to increase both its toughness and HDT.

INTRODUCTION

new technology to improve simultaneously the A heat distortion temperature (HDT) and the tough- ness of (notched Izod) poly(viny1 chloride) (WC) has been developed. It requires the preparation of a binary blend (BB) of two polymers A and B with special mor- phology. Polymer A is a polyglutarimide with a glass transition temperature (Tg) higher than 155°C and Polymer B is an ethylene copolymer. The Polymer B has to be the matrix while Polymer A is the dispersed phase. This morphology is critical to effect complete dispersion of BB in WC.

Usually, plasticizers are added to WC to increase its toughness at the expense of HLYT and of flame re- tardancy (1). Attempts to raise the HDT by adding glass fiber or semi-compatible polymer such as alpha methyl styrene acry1onit.de (AMSAN) (2, 3) are limited by the inherent Tg of the WC. Increasing Tg of PVC by adding miscible high Tg polymers (> 100°C. the higher the better) is currently being applied (4, 5) at the ex- pense of the alloy’s toughness. They are amorphous polymers and require in general a processing temper- ature of 100°C higher than their Tg(s). Imidized acrylic (IMAC) or polyglutarimide is one of these amorphous polymers (5). Depending on the imidization level, Tg could be from 100°C to as high as 165°C.

MATERIALS AND EXPERIMENTAL

Imidized acrylic (IMAC) polymers are marketed by Rohm and Haas under the trade name Paraloid. EXL 4261 grade is a high molecular weight (HMW) fully imidized version with a glass transition (T,) of 160°C. HT510 grade is the low molecular weight (LMW) grade

and is a partially imidized one with Tg of 135°C. Ethylene copolymer (EC) is marketed by E.I. DuPont under the trade name Elvaloy 6249. Commercial PVC, along with lubricants, thermal stabilizers, processing aids, and pigments, is used. For clear W C , pigment is not added. The MW of the PVC is selected based on the K value of 60 and 52 for extrusion blow molding and injection molding grade, respectively. During the compounding step, the materials can be used as pow- der or pellet form. For convenience, BB is made from pellets of HMW IMAC and EC, while WC/BB blend is made from powder. In general, we try to have either full pellets or powder as raw materials during extru- sion or molding step. A Werner & F’fleiderer twin ex- truder with co-rotating screws of 30 mm was used with four individual heating zones. Generally for BB preparation, hand melt temperature is aimed at 260-270°C while for WC/BB, it is about 220-230°C. An injection molding machine of HPM type 6 oz (168 gr) was used. It has four heating zones. Stock melt is aimed at 190 to 195°C.

RESULTS AND DISCUSSION

Because of its decomposition temperature, around 240”C, WC could not be melt processed directly to a compatible high Tg (> 140°C) amorphous polymer such as HMW IMAC. In industrial practice, only LMW IMAC of Tg < 135°C could be added to WC at 230°C to have a modest gain in HDT at the expense of tough- ness, which, in turn, could be improved by adding a rubber or toughening agent at the expense of the HDT (vicious circle). In this process, the nonflammability character is decreased. In practice high HDT, high toughness and V-0 cannot be produced. Therefore, it

30 JOURNAL OF VINYL & ADDlTIVE TECHNOLOGY, MARCH 1998, Vol. 4, No. 1

Super Tough PVC Alloy

would be a process breakthrough in WC properties if we can add industrially a miscible polymer with Tg of 150°C without hurting its toughness and its V-0. We discovered a new proprietary technology where a bi- nary blend (BB) could be added to WC to increase at the same time the toughness and the HDT. This tech- nology has been tested industrially at several PVC compounders and producers worldwide. For instance, a PVC alloy could be made into a super tough mate- rial having a notched hod of 850 J /m (16 ft. lb./in.), a HDT of 100°C and a flex modulus of 2200 MPa (320 Kpsi) compared to pure PVC of 26 (0.5); 70 and 3200 (460), respectively. To our knowledge, this is the first time that PVC could have both antagonistic properties (HDT and toughness) improved at the same time (6).

Phyaioal Characteristics and Morphology of the Binary Blend

The proprietary technology requires the BB to have two Polymers A and B with a specific morphology. The physical characteristics of Polymer A is to be miscible with WC and incompatible or semi-compatible with Polymer B. Miscibility or compatibility is defined as single Tg in the blend. Incompatibility is defined as immiscibility where two distinct Tss of each compo- nent result. Semi-compatibility is defined as partial miscibility. Polymer A should have glass transition higher than 155°C. It must have a common process- ing temperature range with that of Polymer B. The physical characteristics of Polymer B are to be semi- compatible or compatible with PVC. Polymer B can be crystalline or amorphous, with Tg below 100°C (prefer- ably below 0°C) and T, below 180°C. The processing temperature range of Polymer B should encompass that of PVC and of Polymer A. An example of Polymer A is high MW IMAC or polyglutarimide of Tg 160°C with processing temperature range from 250°C to 300°C. An example for Polymer B is an ethylene copolymer (EC) with processing temperature range from 190°C to 280°C.

During the formation of the BB at melt temperature between 260 and 270"C, the morphology requires that Polymer A be the dispersed phase and Polymer B to be the matrix. This requirement can be satisfied easily with small amount of Polymer A (i.e. 15% and B at 85% by weight) but much more difficult with large amount of Polymer A (i.e. 85Yo). To increase HDT, a large amount of Polymer A is needed. The reason is that only Polymer A contributes to the HDT of the PVC, while Polymer B is for the toughness (low stiff-

ness). The resulting BB with the right morphology, which is Polymer B "reinforced" with Polymer A, could be then melt processed with W C at its usual process- ing temperature from 190°C to 230°C. The resulting WC/BB alloy shows a much higher Tg (due to misci- bility of PVC and Polymer A) and toughness (due to Polymer B) .

From Table I , three examples (2 to 4) having the same composition of matter but using different tech- niques of melt mixing show totally 3 different results in terms of commercial practicality (skin) and HDT. Example 1 is pure PVC for illustration. In Example 2 (Table I ) , BB is made of 25.5 parts of Polymer A or HMW IMAC and 4.5 parts of Polymer B or EC. This BB has the right morphology: that is, the EC is the matrix. Note that the matrix is the minor part.

The PVC is then melt mixed with this BB at 220- 230°C and shows a drastic improvement in the HDT (from 60 to 88°C). Moreover, judging from the skin of the melt strand and the molded part, it is a homoge- neous mixture (no particulate). In Example 3, the blend AB has similar composition of matter as that of BB, except with the inverted morphology where HMW IMAC is the matrix and the EC is the dispersed phase. During melt mixing with the WC at 220-23OoC, some of the "rubber toughened" HMW IMAC pellets have not been fully dispersed in PVC. As a result, the skin of the melt strand is rough with undispersed HMW IMAC pellets and the resulting HDT of the PVC alloy is not as high as in Example 2. From a commercially practical point of view, this WC alloy is not accept- able. In Example 4, the HMW IMAC, EC and PVC are melt mixed in one step at 22G230"C. No continuous melt strand could be obtained because of the hetero- geneity of the systems at the extruder die. In fact at 230°C. the IMAC is hardly processable as is. The be- havior of the HMW IMAC pellets could be explained as in Example 3 above. From Table 1 the results demon- strate that the invented technology is unique to im- prove the HDT and the toughness.

Effect of the BB Alloy on the Tg of the PVC Woy

The technology is to increase the glass transition (Tg) of the PVC alloy. HMW IMAC is totally miscible with PVC. A plot of Tg (at mid point) of the PVC alloy versus the amount of BB shows that it is linearly pro- portional (Rg. I ) . A straight line derived from the data indicates that each percent (by wt) of the BB increases the Tg of the WC alloy by 0.74"C. Rgures 2 and 3 show the Tg (from DSC) and the E' and E" (from DMA)

Table 1. Effect of Mixing Protocol on the HDT and the Skin of the PVC Alloy.

PVC BB Blend AB IMAC EC HDT Example ("A) ("/.I (%) ("/I @I ("C) Skin

1 100 0 2 70 30 3 70 0 4 70 0

0 0 0 60 Smooth 0 0 0 88 Smooth

30 0 0 85 Rough 0 25.5 4.5 NIA Rough

JOURNAL OF VINYL &ADDITIVE TECHNOLOGY, MARCH 1998, Vol. 4, No. 1 31

Gia Huynh-Ba

I

GLASS TRANSITION VS K DUPOKT MODIFIER Y = 81.528 + 0.74187~ R"2 p: 0.997

3 " I 150

0 10 20 30 40 SO SO 7 0 80 00100

Flg. 1. Tg us. w% of binary blend. % WT DUPOIJT MODIFIER

of the pure PVC and the 70/30 (PVC/BB wtYo), re- spectively. It is very apparent that the technology pro- duces a new PVC alloy with higher Tg and higher modulus retention at higher temperature.

Comparison Between BB and HT5 10

LMW IMAC can be used effectively to increase the HDT of PVC. Comparing the BB and LMW IMAC data showed that for each percent of IMAC, the use of BB is better than LMW IMAC (Figure 4: curve BB vs. LMW IMAC). For instance at 30% IMAC, our technology

could produce an HDT of 95°C versus 90°C for LMW IMAC. In our technology with HMW IMAC as the dis- persed phase, the IMAC domains, which are in micron to sub micron size, require much less energy and temperature to make them totally miscible with W C during the extrusion step. Experiment shows that in a direct melt mixing of the same HMW IMAC with PVC, the extruded strand has a very rough skin, and the resulting HDT is lower than that of our technology. The data could be explained by the incomplete disper- sion of the HMW IMAC in WC. In fact the HMW IMAC pellets required temperature much higher than 250°C to be melt processable. On the other hand, adding LMW IMAC with Tg about 135°C to PVC decreases the toughness as shown in Fig. 5 [curve: LMW IMAC). For comparison, the BB at the same level maintains the PVC alloy toughness (Fig. 5; curve: BB). Thus, to main- tain the WC toughness it is necessary to add some rubber to the LMW IMAC systems which will eventu- ally decrease further the HDT as well as the flamma- bility (V-0). Therefore, for a high HDT/toughness/V-0 combination, the BB approach would be better than the LMW IMAC. Patterson also reported similar re- sults while using Lh4W IMAC (5).

Effect of the Polymer B Chemical Structure

One of the physical criteria of the Polymer B is to be semi-compatible or compatible with PVC. Ethylene copolymer (EC) is a good example as Polymer B. With certain comonomers such as butylacrylate and car- bon monoxide (CO), the EC is miscible with the WC.

- 0 . ; ,

c I

J 0

n' 83.s-c x

B I OUPO

Fig. 2. DSC of pure PVC and PVC alloy.

32 JOURNAL OF VINYL 81 ADDlTlVE TECHNOLOGY, MARCH 1998, Vol. 4, No. 1

Super Tough PVC Alloy

2.s 300

-250

-206 - ; i *&I

iooi:,

n-

a I S

; '

PI

- 1.0-

-so

- 0

0.5-

0.0-

1 20 40 6b d ldo lh lio l&

Tawrcature 1 2 ) D)u r3.W Hg. 3. DMA of pure W C and PVC &y.

W W [I:

0 8 CI

11O- l i

w + - = - - * - - - - - ' 0 10 20 30 40 6 0 6 0

BB or LMW IMAC

Fig. 4. HDTus. wt96 of BB

PVC (K-54)

l o + - ' - * - - - ' - - ' 0 10 2 0 30 40 SO $ 0

BB or LMU IMAC

Fig. 5. hod us. wf96 of BB us. LMW IMAC.

Table 2. Effect of BB Type on HDTIToughnesslStiffness and Clarity.

Type of B Rubber PVC BB in BB ("/I HDT N. lrod Flex Mod

Example (Yo) (W ("/.) CIS- ("C) (Jim) (Mpa) Clear ~~ ~~~

1 100 0 NIA 10 71 1270 2750 Yes 2 70 20 ELV' 10 88 960 2500 Yes 3 70 20 E-1" 10 96 1170 2600 No 4 65 25 ELV" 10 96 635 2500 Yes

'ELV = E/BA/CO.60/30/10 wt.56 "E-1 = EIBNGMABB 8/28/52 wt.%

" W S = Codshell toughener.

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Gia Huynh-Ba

Table 3. Mechanical Properties of PVC Alloy Using BB Technology. ~

BB' CIS'+ Flex Mod H DT N. lzod

1 0 0 3200 70 26 Yes 2 25 8 2000 89 600 Yes 3 30 20 2200 1 00 850 v-1

Cases ("4 ("4 ( M W ("C) (Jim) v-0

~

'BE: Binary blend from DuPont "CIS = Core/shell toughener KM333

As a consequence, the PVC alloy could be made into a clear and tough material. With other comonomers, it is semi-miscible. The EC can rubber toughen the W C with small discrete particles. As a consequence, the W C alloy is opaque. To further increase the tough- ness of the PVC alloy, acrylic core shell could be added. Matching the right refractive index or not to the clear or opaque WC alloy produces a tough/stiff/ V-0 material. Table 2 shows the effect of the chemical structure of polymer B in the BB on the properties of the WC alloy. For example, adding 20% of BB made of B compatible with WC such as E/BA/CO, the W C alloy is clear, but the HDT is only 88°C (Ex. 2, Table 2). while using a semi compatible B, the HDT is 96"C, but the WC alloy is opaque (Ex. 3). To increase the HDT of Clear W C alloy to 96"C, 25% of BB is needed (Ex. 4). Note that the toughness and stiffness are still very high: 630 J / m and 2500 Mpa, respectively. Table 3 shows that toughness, stiffness and HDT of Opaque PVC (Molding grade) can be improved while flamma- bility (V-0 and V-1) is maintained. Example 2 shows that toughness can be improved 2 3 X while HDT is improved from 70 to 89°C. Further improvement both

in toughness and stiffness can be seen in Example 3 (Table 3) to 3 3 X and lOO"C, respectively.

CONCLUSION

Our technology using HMW IMAC and a special morphology provides better HDT and toughness com- bination than conventional LMW IMAC. The morphol- ogy of BB is the critical step of the invention. That is, the IMAC has to be the dispersed phase. This propri- etary technology could be applied to other plastics having problems similar to those of WC.

REFERENCES 1. D. L. Dunkelburger and J. T. Lutz, lmpact Modzfims for

PVC: The Histoy and practice. Wiley, New York (1992). 2. N. A. D'Souza, A. Letton, and A. R. Elliot, J. Vinyl A d d

Tech, 1, 88 (1995). 3. Plastics Technology, 14 (July 1994). 4. L. M. Robeson, SPE FtETEC, PVC Blends and Alloys and

5. R. A. Einhorn and J. R. Patterson, J. VhyL Tech, 13, 60

6. U. S. patent 5,502.1 1 1 assigned to E.I. DuPont.

Graft Polymers, 134 (1989).

( 1991).

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