Polymer Testing 6 (1986) 361-367

A Microhardness Test to Detect Crystallinity Changes in Stabilized Rigid PVC

A. Gonz~ilez, t B. M a r t i n , t M. Mufioz~t and J. A. de Sa j a t

tFacultad de Ciencias, Laboratorio de Flsica del Estado S61ido, Valladolid, Spain Rio Rodano S. A., Miranda de Ebro, Spain

SUMMARY

Vickers microhardness testing has been found to be a sensitive method to detect small crystallinity changes in industrial mass- polymerized PVC. In this paper we provide experimental evidence on the influence of thermal treatment (quenching and subsequent annealings) on the crystallinity and consequent surface hardness of solid sheets of stabilized PVC.

1. INTRODUCTION

For a long time poly(vinyl chloride) (PVC) was regarded as an amorphous polymer until the X-ray diffraction experiments of Natta and Corradini showed that, on a special polymerized PVC com- pound, crystalline regions exist with a syndiotactic orthorhombic structure and unit cell parameters a = 10.6/~, b = 5.4/~ and c = 5.1A. 1

Recent studies show that the degree of crystallinity of industrial PVC is rather poor and depends mainly on the proportion of syndiotactic sequences existing in the compound. 2-4 The syndiotactic-isotactic ratio of this polymer and consequently its crystallinity depend on the polymerization conditions and on the thermal and mechanical history of the product. In conventional industrial PVC this degree of crystallinity is below 15%. 5

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362 A. Gonzdlez et al.

As far as the crystalline phase is concerned it can be assumed that, being the cause of the high stability of the microdomains (10-20 nm) existing in the primary particles (1-2#m), 6-s these regions are responsible for the mechanical and thermal stability of the final product. The recognition and understanding of the way in which PVC crystallizes must lead to a better utilization in their present and new areas of application.

In polymers with a high degree of crystallinity it is possible, in principle, to determine crystallinities by means of X-ray diffraction (WAXD, SAXD, and SAXND), spectroscopic (IR and Raman), electron microscopic and calorimetric (DSC) measurements, but such techniques are complicated and their industrial use is rather problematical.

Earlier investigations in our laboratory 9-11 have proved that Vick- ers microhardness (MHv) testing is a very interesting non-destructive tool for the study of structural transformations in solid materials and we thought that this simple method could be useful in the evaluation of the changes of crystallinity which occur in PVC under significant thermal treatments.

It is the aim of this work to study the possibility of using this test to detect the small crystallinity changes induced by thermal annealing of industrial PVC samples.

2. EXPERIMENTAL

2.1. Materials

Three commercial mass-polymerized resins supplied by Rio Rodano (Spain) with average molecular weights of 45 000 (a), 52 000 (b) and 78 000 (c) were used in our experiments. The samples were formu- lated with 15 phr octyl tin mercaptide as a thermal stabilizer. Each sample was prepared by milling at 150 °C (5 min) and pressing at 160 °C (1 min; 107 Nm -2) to form 20 cm x 20 cm plates 1.5 mm thick. In an attempt to minimize existing crystallinity and to ensure a similar thermal history, all samples were simultaneously heated to 180°C (2min) and then quenched in ice-water. These times and temperatures were the highest that could be used without visible appearance of thermal degradation. Samples were subsequently

Microhardness test to detect crystallinity changes in rigid PVC 363

annealed for 30, 60 and 90 min at various temperatures between 20 ° and 160 °C and then cooled slowly to room temperature. Finally the heat-treated plates were cut into sheets approximately 2 cm x 2 cm, optimal dimensions for the MHv measurements. Annealing times of 60 and 90min were carried out only in the interval 20-140°C; treatments at higher temperatures for these times lead to the risk of producing thermal degradation.

2.2. Apparatus

MHv measurements were obtained at room temperature using a hardness tester with a square-pyramidal indenter. Values were calculated by computing the ratio of peak contact load to the projected area of the impression according the equation:

P MHv = 1819.1 x 107 (d + D) 2 (Nm-2)

where P is the load applied perpendicularly to the sheet, in grams, and d and D the diagonals corresponding respectively to the plastic and elastic contribution to the indentation, in microns. Apparatus and procedures were basically the same as those employed in earlier papers. 9-11 Optimal applied load and time of application were calculated as 60 g and 30 s respectively.

Thermal treatment under an atmosphere of nitrogen was carried out in a thermal programmed oven.

3. RESULTS AND DISCUSSION

A graph of MHv versus annealing temperature (T~) for the three resins is shown in Fig. 1. This representation indicates that heat treatment yields four different types of behaviour, indicated on the graph with roman numerals. In the first, there appears a rapid linear hardening of the sample with Ta, associated with an initial packing or arranging or the polymer chains; this region ends at a temperature of around the glass transition (Tg) of PVC. Annealing above Tg and up to approximately 100 °C produces only a slow hardening. In treat- ment above 100°C important variations were found; in this third region the MHv reaches its highest value at 110 °C, falling thereafter

364 A. Gonzdlez et al.

14

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I"I 2030 46 5~0 60 70 8() 910 1()01110 120 130 140150160

annealing teml~ratur~ Fig. 1. Variation of microhardness with annealing treatments in samples with M , = 45 000 (a), Mw = 52 000 (b) and M, = 78 000 (c). MHv of quenched samples

prior to annealing was 11.9 x 10 -7 Nm -2.

to the values found for the lower annealing temperatures (samples a and b) and even lower for sample c.

The behaviour observed in the temperature interval 70-160 °C is similar to that obtained previously by different authors from X-ray diffraction, 12,13 infrared, 14,15 tensile yield stress 13 and thermal analysis ~3,~6,~7 experiments, in PVC samples submitted to equivalent thermal treatments. All these authors attribute their results to changes in the crystallinity level of the polymer with a maximum order always when the samples were annealed at 110 °C.

Our results also show that the sample polymerized at a lower temperature (higher molecular weight) presents a superior crystal- linity except at the higher annealing temperatures where the effect of thermal degradation is evident; from Ta-140°C the sample c displays a very slight discoloration. These results are consistent with general data in the literature ~849 so that a decrease in the polymeriza- tion temperature increases not only the molecular weight of the PVC but also its syndiotacticity and therefore its crystallinity. The be- haviour of sample c is also coherent with previous results of this

Microhardness test to detect crystaUinity changes in rigid PVC 365

laboratory concerning the decrease of the superficial microhardness in the first stage of thermal degradation of tin-stabilized PVC resins. 20

No significant variations were found for the samples annealed for times of 60-90 min in the interval 20-140 °C.

Our measurements seem consistent with the mass-polymerized PVC model based on the existence of two structures of crystallites, with a wide size distribution and embedded in an amorphous matrix, 21 as well as on the assumption that the hardness of the ordered or crystalline phase is greater than that of the amorphous phase. Of the two crystalline structures one is of a ribbon-like morphology, formed during the second stage of the mass process and with melt temperatures ranged between 120 ° and 200 °C; the other, with a fringed-micelles configuration, is formed during the pre- polymerization stage and begins to melt above 140 °C.4

Annealing treatments probably induce:

(i) a better packing or ordering of the polymer chains in the pseudonematic amorphous phase and formation of a network in which the microcrystals, previously formed, act as tie points;

(ii) a crystal ripening through gradual melting of the smaller and less-perfect crystals followed by the growth of the biggest;

(iii) a gradual melting-out of the developed ordered structure (for temperatures above 110 °C), firstly of the lamellar ribbon-like crys- tallites and subsequently of those with fringed-micelles configuration.

The restructuring and trend to uniformity in size of crystallites, characteristic for each T~, lead to a gradual hardening of the material. The greatest morphological changes occur during the treatments corresponding to the regions I and III, separated by a region (II) in which the rubbery or viscous changes associated with Tg attenuate the elastic response of the crystalline phase. The drop of MHv values detected above 110 °C (region IV), attributed to a melting process, is supported by the reduction of crystallinity according to X-ray data obtained by Kockott 12 and IR experiments of Biais et al. 4

The decrease observed in the MHv values corresponding to the higher annealing temperatures is possibly the result of a loss of crystallinity combined with the effect of the start of thermal degradation of the compound. In the case of sample c this decay is faster as a consequence of its higher crystallinity, which prevents to some degree the ingress of the stabilizer into the primary particles. 22

366 A. Gonzdlez et al.

Important questions remain unanswered and a detailed correlation between MHv results and the data obtained by all other available physical methods is necessary. It is clear that the fundamental development and future importance of this technique will be sub- ordinate to their interrelation with other microscopic methods, mainly with the X-ray diffraction techniques. From a more practical viewpoint, however, this research has shown that the microhardness test is a very useful and sensitive tool for the study of levels of crystallinity, sample comparison and determination of crystallinity variations.

It is not feasible to extrapolate these results to anticipate the applicability of this technique to crystallinity studies in other conven- tional polymers; however, it is reasonable to suppose this will be possible.

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