The influence of calcium carbonate on rigid PVC low-foaming products

I. Overview

Rigid PVC low-foam material is an ideal substitute for wood in terms of its density, performance, secondary processing (planing, sawing, nailing, bonding, etc.), and thermal insulation, sound insulation, moisture resistance, insect resistance, and flame retardant properties. Its formula is composed of PVC (K value 56-60), stabilizer, foaming agent, foaming regulator, nucleating agent, etc. After mixing, it is mainly extruded in the form of skin foaming (Celuca). Among them, nucleating agents are generally inorganic fillers with smaller particle diameters, such as calcium carbonate (CaCO3) and titanium dioxide. Due to the high cost of titanium dioxide, it is generally only used for coloring or outdoor UV protection. And because CaCO3 has low price, wide sources and good stability, adding it to the product can not only reduce the cost, but also act as a nucleating agent. Therefore, in the low foaming processing of rigid PVC, CaCO3 is widely used, and its addition amount varies according to the specific requirements of the product, the formulation system and the performance of the CaCO3 used. There are two kinds of CaCO3 commonly used in the processing of rigid PVC products, namely light CaCO3 and heavy CaCO3. The former is made by chemical methods, and the latter is made by physical methods such as crushing and grinding. In China, the processing and processing technology of lightweight CaCO3 is relatively mature, therefore, more lightweight CaCO3 is used. In Europe, the development and research of heavy CaCO3 has taken a long time, and a relatively mature processing and processing technology has been formed. Therefore, the use of heavy CaCO3 is more common than light CaCO3. This article focuses on the impact of CaCO3 on the foaming effect and processing performance of rigid PVC skinned low-foaming extrusion products.

II. Test part

1.       The influence of CaCO3 on the foaming effect

1.1 The influence of CaCO3 varieties on the foaming effect

Choose 3 different types of CaCO3 for extrusion test. The test formula is a lead salt stabilizer system, and the basic formula is shown in Table 1. Except for the different varieties of CaCO3, the other ingredients and the added amount of the three formulas tested remained unchanged, limited to the same test equipment, die and process. The three types of CaCO3 are domestic light calcium (A), imported heavy calcium (B) and domestic heavy calcium (C). The average particle size of the three types of CaCO3 is 2μm, and they are all coated with stearic acid.

Table 1

Through comparative experiments, when extruded with light calcium and imported heavy calcium, the foamed sheet has a smooth surface, uniform skinning and color (see Figure 1a, Figure 1b), moderate surface hardness (above HD65), and uniform cross-sectional cells; and When the domestic heavy calcium formula is extruded, the surface of the sheet is uneven, and some parts have smooth surface, good skinning, and normal hardness, some parts have light color, more surface holes, and bad skinning (see Figure 1c). The hardness is not enough (HD is lower than 30), and the cross-section cell is not uniform. Through the analysis of the light-colored parts, it is found that there is no dense crust layer, the surface density is low, the relative content of pigment is less, and the color becomes lighter. The principle of the surface skinning of the Celuca method is that after the material melt is extruded from the die, it immediately enters the cooling setting sleeve for cooling and setting. (Dense layer). It is inferred from this that under normal extrusion and cooling conditions, the surface layer of the product is not skinned because the corresponding melt has already formed bubbles (also called pre-foaming) before the extrusion die. At this time, no matter how it is cooled, its It is also impossible for the surface layer to form a crust layer.

Figure 1 The effect of different CaCO3 on the surface crust

a-domestic light calcium; b-imported heavy calcium; c-domestic heavy calcium

The factors of pre-foaming include: too little foaming regulator, too high processing temperature, too long residence time of the material in the barrel, and too little head pressure. In the formula of the comparative test, only the variety of CaCO3 was changed, and the other conditions were exactly the same. As a result, there was no skinning or bad skinning on the surface of the product. Therefore, the reasons for pre-foaming can exclude factors such as the amount of foaming regulator added or processing technology. In order to further analyze the root cause of pre-foaming, it is necessary to clarify the nucleation mechanism of CaCO3 in PVC foaming. The foaming process of plastic products goes through three stages, namely the formation of the bubble core, the growth of the bubble, and the shaping of the bubble. Because CaCO3 transfers heat quickly in the material, it is easy to disperse into the melt to form hot spots. These hot spots can reduce the viscosity and surface tension of the local area, so that when the blowing agent decomposes, it is easy to adsorb gas to form bubble nuclei, and the external pressure is large enough or When the melt strength is large enough, the bubble core will not expand in the melt. When the external pressure drops or the melt strength decreases, the bubble will expand and expand, so that the internal pressure of the bubble balances with the external pressure and melt strength. That is to say, even if the pressure of the die remains unchanged, after the strength of the melt is reduced, it will foam in advance. Therefore, the reason for the pre-foaming of the domestic heavy calcium formula is due to its poor coating treatment, poor dispersion in the material, formation of aggregates, local overheating, and lower melt strength and pre-foaming.

1.2 The influence of CaCO3 particle size on foaming ratio

Replace the CaCO3 in Table 1 with different particle sizes of the same variety. The average particle sizes are 013μm, 113μm, 310μm, and 510μm. Under the same test conditions, extrusion is carried out at three screw speeds, and the particle size and expansion ratio are analyzed. relation. The test results are shown in Figure 2.

Figure 2 The relationship between the particle size of CaCO3 and the expansion ratio

CaCO3 particle size: 1-013μm; 2-113μm; 3-310μm; 4-510μm

It can be seen from Figure 2 that among the selected particle sizes, under three different screw speed conditions, when the CaCO3 particle size is 113 μm, the expansion ratio of the sample is the largest, and the effect is the best. When it is larger or smaller, the expansion ratio decreases instead. This is because when the particle size of CaCO3 is larger, the number of particles per unit mass is smaller, and the "hot spots" (that is, nucleation points) dispersed in the melt are also fewer; and the thicker the particles, the gas is not easy to form around it. Bubble core. However, if the particle size of CaCO3 is too small, it is easy to gather together and form coarse particles, which affects its dispersion and reduces the bubble nucleus. In addition, some scholars have shown that [4]: When the amount of foaming agent is constant, the concentration of nucleating agent increases by 10 times, the number of bubbles will increase by 4 times, and the volume of each bubble drops to 1/50 of the original. Therefore, the effective amount of nucleating agent is the key factor that affects the foaming ratio of the product.

1.3 The influence of the amount of CaCO3 added on the expansion ratio

Fix the other components, proportions, and the variety and particle size of CaCO3 in Table 1, and only change the amount of CaCO3 added, which are 1phr, 3phr, 5phr, 7phr, 9phr, and 15phr. Under the same processing conditions, the extrusion is carried out at 3 screw speeds. A test was conducted to analyze the relationship between the amount of CaCO3 added and the expansion ratio. The average particle size of CaCO3 used in the test is 2μm, and the result is shown in Figure 3.

Figure 3 The relationship between the amount of CaCO3 added and the expansion ratio

Screw speed: 1-30r/min; 2-45r/min; 3-60r/min

It can be seen from Figure 3 that when the amount of CaCO3 added is 5 phr, the expansion ratio of the sample is the largest. When the amount of CaCO3 added is small, it does not have enough nucleation "hot spots" in the melt, so that more bubbles cannot be formed, and the foaming ratio of the product is low; when the amount of CaCO3 is too much, although the CaCO3 has more particles , Can form more bubble cores, but due to the poor compatibility of CaCO3 with PVC, the melt strength is greatly reduced. As a result, the foaming ratio of the product is reduced.

2. The effect of CaCO3 addition on processing performance

The test formula is shown in Table 2, where CaCO3 is stearic acid coated light calcium, with an average particle size of 2μm. A torque rheometer (brabender, model W50EHT-3Zones) was used to analyze the effect of CaCO3 addition on processing performance. The torque rheological test conditions are: 160℃, speed 35r/min, adding amount 60g, the test results are shown in Figure 4.

Table 2 Processing performance test formula (phr)

PVC (K value 58) composite lead stabilizer foaming regulator composite foaming agent calcium carbonate

100 6 6 1.5 0

100 6 6 1.5 3

100 6 6 1.5 6

100 6 6 1.5 10

100 6 6 1.5 15

Each torque curve in Figure 4 has two peaks. The maximum torque produced by the decomposition of foaming agent). With the increase of CaCO3, the minimum torque, the peak of plasticizing torque and the peak of foaming torque all show a downward trend, but the balance torque is basically the same.

Figure 4 The influence of CaCO3 addition on processing performance

This is because CaCO3 is coated with stearic acid. Its particle size is only 2μm, which is much smaller than PVC particles. The compatibility between inorganic CaCO3 and organic PVC is very poor. When the materials are in solid form, CaCO3 has similar external lubrication. The effect of this can make the large particles of PVC easy to slide and reduce the torque. Generally speaking, CaCO3 will delay the plasticization of materials. A lot of foaming regulator ACR is added to the formula. ACR can increase the viscous fluidity of the material, improve the melt strength, and speed up the plasticization speed of the material. The more ACR is added, the stronger the effect of promoting plasticization, which offsets the effect of CaCO3 on plasticization, and the material quickly reaches the plasticization stage. As the material is plasticized and the temperature of the material rises further, the foaming agent decomposes rapidly, and the viscosity of the melt increases again. As the foaming agent is completely decomposed and the materials are evenly mixed, the extrusion tends to be stable and the torque tends to be balanced.

3. Conclusion

(1) The coating quality of CaCO3 is not good or the dispersibility is poor, and it is easy to produce local pre-foaming, so that the surface of the corresponding part cannot be skinned;

(2) When the particle size of CaCO3 is about 1.3μm, the expansion ratio is more ideal, and the density of the product can be reduced at the same time;

(3) Lightweight CaCO3 with a particle size of about 2μm and coated, when the addition amount is 5phr, the ideal expansion ratio and product density can be obtained;

(4) In the PVC low foaming formula, when the content of CaCO3 is in a certain range, it has little effect on the plasticizing time and foaming time, but it has a certain effect on the torque of plasticizing the material.