Thermal degradation of polymer concrete

Thermal degradation of polymer concrete Jochen Marschall and Frederick Milstein

Departments of Mechanical Engineering and Materials, University of California Santa Barbara USA.

Abstract

Polymer concretes (in which polymers, rather than cements, are used as binders of aggregate materials) are increasingly used in a wide variety of applications. In this paper, a relatively simple experimental procedure for evaluating the thermal degradation of polymer concrete is described and the results of testing two distinct polymer concretes (one based on methyl methacrylate and the other on a polyester) are presented. Thermal degradation was determined by measuring the weight loss experienced by samples exposed to various temperatures for different times. In addition, each sample was given a subjective structural integrity rating on a scale of 'A' to 'D' (where 'A' and 'D'. respectively, indicate no observed loss o f structural integrity and complete loss of cohesion). It was observed that, for given times of exposure (t), there exist well-defined "characteristic' temperatures, T(t) and T.(U. respectively, below which no weight loss or mechanical degradation was observed. For the methacrylate based polymer concrete, exposed to elevated temperatures for two minutes, no loss of strength was observed below ~(2) ,.~ 680"F and zero weight loss w a s measured below TJ 2 ) "~ 600:F. When the exposure time was raised to ten minutes, both characteristic temperatures (i.e. ~(10) and TJIO)) dropped to about 520~ F. For the polyester based polymer concrete, after a two minuLe exposure, no loss of strength was found below ~(2) '~ 740~F and no weight loss was observed below T,.(2) ~ 620 F.. after a ten minute exposure, ~(1 O) and T ( I O) dropped to about 610<'F and 540"F, respectively.

Introduction

Polymer Concretes (PC) are finding increasing use as construction, repair, and replacement materials for a wide variety of applications; examples include bridge construction m, the manufacture of power pole components ~2~,and the patching of roadways ~35~ and runways~67~; they have also been used in highly corrosive geothermal environments ~ " and as a repair and replacement material for porcelain electric insulators '2 ~5~. PC is a composite material consisting of a continuous matrix phase (which is essentially a mortar comprised of polymerised m o n o m e r s and fine powder-like aggregates) and larger-sized a g g r e g a t e materials. Monomers used in PC include epoxy resins, polyester resins, methyl methacrylate (MMA), furane derivatives, and styrene, while aggrate materials may be rocks, sand, ceramic powders, glasses, metallic fibres, etc. Typically, the weight of organic c o m p o n e n t s in PC is about 10 per cent. The mix generally includes a catalyst (which initiates the polymerisation process), a promoter (which acts as a crosslinking agent), and a coupling agent (which enhances binding between the organic m o n o m e r s and inorganic aggregates). PC is especially useful for applications such as highway and runway repair, because it develops a

14

high strength fairly quickly. For example, regular Portland cement concrete (with a water/cement ratio of 0.6 to 0.8) has a 7-day strength of about 70 per cent of its 28-day strength, whereas PMMA (polymethylmethacrylate) based PC develops about 80 per cent of its maximum strength after one day (the maximum compressive strength is about 20,000 - 30,000 psi, reached after about 7 days) v6~. In addition, PC possesses relatively high strength in tension (about 3000 psi for PMMA based PC'6~); when compared with regular Portland cement concrete, it is lighter, less prone to crack and void formation during curing, and more resistant to chemical attack. A possible disadvantage of PC is that the polymer based matrix phase can degrade at elevated temperatures; thus it is useful to have a means of determining such potential degradation quickly and easily for a wide variety of PC materials. The present paper describes a relatively simple experimental procedure for evaluating the thermal degradation of PC and presents the results of tests with two distinct PC materials (one based on the thermoplastic PMMA and the other on a thermoset polyester resin). A preliminary series of tests indicated that both polymer concretes degraded

through gradual sublimation and charring as they were exposed to increasing temperatures. (One might have expected different behaviour from the PMMA PC since thermoplastics gradually soften and eventually melt as temperature is raised; the observed behaviour is evidently a result of the processing, which includes the dispersal of the fine aggregates to form a mortar and the addition of chemicals to cause tight bonding between the PMMA and the fine aggregate dispersion in the mortar phase.) The initial experimental results also showed that shorter exposures to higher temperatures could produce the same loss of cohesion in a sample as longer exposures to lower (elevated) temperatures, thus indicating reaction rates that are temperature dependent. At sufficiently high temperatures (and after sufficient time), loss of cohesion, and hence of structural integrity, occurred because the organic media binding the aggregates had charred or sublimated, leaving the sample with the consistency of a 'sugar cube', which could be easily broken or crumbled by hand. In view of the results of the preliminary tests, it was considered appropriate to characterise the thermal degradation behaviour of the polymer concretes in two ways, ie.

C O N S T R U C T I O N & B U I L D I N G M A T E R I A L S V o l . t No. 1 M A R C H / A P R I L 1987

(1) by measuring the weight loss experienced by samples exposed to various temperatures for different times and

REMOVABLE STEEL TUBE

(2) by giving these samples subjective ratings indicating their structural condition after thermal exposure (as is explained in the following section). Primary consideration in the design of the experiments were (i) to provide for quick and uniform heat~ng of the polymer concrete samples, (ii) to maintain a constant exposure temperature during each test run, (iii) to provide for rapid removal of each sample from the heating environment after a desired exposure time, and (iv) to use generally available laboratory equipment and/or f'~tures that can be readily produced.

Experimental procedure Quick and uniform heating of the polymer concrete samples was achieved by heating thin, fiat samples (of no more than 0.2 inch thickness) in a Lindberg Type 56622 crucible furnace, which was regulated using a Lindberg Type 59344 temperature controller. A cast iron test block containing a steel tube insert was placed in the furnace and preheated to a desired temperature; the block provided a thermal mass that was insensitive to convective temperature fluctuations within the furnace and thereby enabled a relatively constant sample exposure temperature to be maintained. The tube (into which the specimens were placed) could be removed quickly at any time, and thus provided a means for rapidly removing the specimens from the furnace. The set-up is illustrated in Figure 1. An access hole was drilled into the test block in order to insert a thermocouple which was used to monitor the temperature continuously. The thermocouple employed was a used in conjunction with an Analog Device Type K Digital Thermometer. The composition of the two PCs that were tested are listed in Tables 1 and 2. The PMMA based PC was produced at UCSB as part of ongoing research into the physical properties of polymer concrete (eg. tensile and compressive strength, fatigue life, and adhesive behaviour). It is 13 per cent organic and 87 per cent inorganic aggregate, by weight,

TO DIGITAL THERMOMETER

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Fig1

Test block set.up. Components are shown to scale (tube is 1 in. o.d)

Table 1. Composition of PMMA based PC

Ingredients A.

B.

Inorganic Components (87%) Wedron 4098 Wedron 4030 Silica 70 Silica 290 Hydrated alumina 15 #m min-u-sil Ti02 Organic Components (13%) MMA (Methyl methacrylate) A-11 (Polymethyl-methacrylate) (PMMA) X-980 (TMPTMA) (Trimethyl-propane-trimethacrylate) A-174 (Silane) BPO (Benzoyl peroxide) DMpT (N,N.-dimethyl-p-toluidine)

CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 1 M A R C H / A P R I L 1987

Weight (%)

31.84 12.18 12.18 14.61 9.74 4.87 1.56

10.87 0.57 0.57 0.57 0.21 0.21

15

I

Ingredients Weight (%) A. Inorganic Components (88.5%) 4 Q ROK 26.2 Sand (3/16 x 1O) 23.6 Sand (1/16 x 1/8) 17.0 Silica flour 16.9 Hydrated Alumina 4.0 TiO 2 0.6 Black Pigment 0.07 B. Organic C o m p o n e n t s (11.5%) P o l y e s t e r Resin Silane A-174 MEKP Hydroquinone

with the aggregate being primarily c o m p o s e d of fine silica sands. The second type of concrete was produced commercially; it is 11.5 per cent organic and 88.5 per cent inorganic aggregate. The test samples were cut to size using a diamond cutoff wheel. Two series of tests were run for each type of polymer concrete; one at a two-minute exposure time and the other at a ten-minute exposure time. The first series comprised 18 different tests and the second series consisted of nine different tests (for each type of polymer concrete) for a total of 54 tests. The following procedure was adopted for each test run: 1 ) The crucible furnace was heated to the desired temperature with the test block and steel tube insert inplace on the base of the furnace. The range of temperatures used in the test procedure was sufficient to include temperatures high enough to cause the s p e c i m e n s to crumble 'by hand' and low enough so that no noticeable loss of strength was observed 2) When the digital t h e r m o m e t e r indicated that the test block and steel insert had stabilised at the desired temperature, a sample of polymer concrete was placed into the steel tube using tweezers 3) After the desired exposure time (two minutes or ten minutes), the steel tube was withdrawn from the test block (and from the furnace) with a pair of pliers and the s a m p l e was

16

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PMMA PC

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immediately removed from the tube 4) The steel insert was then replaced in the test block and the entire process was repeated, with another sample, for the next test temperature and time of exposure. The samples were weighed (before

and after each test) on a Mettler PE400 electronic scale. After the final weighing, each sample was examined and given a structural rating based on a subjective scale wherein a rating of 'A" implies 'no apparent loss of structural cohesion'; a 'B' rating

CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 1 MARCH/APRIL 1987

indicates that the 'corners of the sample could be chipped with effort'; samples rated 'C' 'could be broken readily by hand'; and 'D' samples 'could be crumbled by hand.'

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Results and c o n c l u s i o n s rr ] IOmin / /2 min The test data for the PMMA based polymer concrete are displayed in Figures 2 and 3 and the data for the polyester based polymer concrete are shown in Figures4 and5. Figures 2 and 4 show the variation of the structural rating with two and ten minute exposures to various tem/ ITs(lO) ITs(2) peratures and Figures 3 and 5 show | I I iI I I i the variations of weight loss with these 40O 60O 8OO I000 exposures. The average exposure temperature experienced by each TEMPERATURE (°F) sample was used for plotting purposes. (The set-up generally enabled Fig 4 Structural rating of polyester resin based PC following 2 rain. and the temperature to remain constant 10 min. exposures to elevated temperature to within about +5°F.) A survey of the results shows that, I I I I I " I for given times of exposure (t), there I0" exist 'characteristic' temperatures, POLYESTER PC • y -I TJt) and T,(t), respectively, below which no weight loss or mechanical degradation was observed. For the PMMA PC exposed to elevated temperatures for two minutes, no apparent loss of strength was observed below T~(2) ~ 680°F and zero weight loss was measured below TJ2) "~ 600°F. When the exposure time was IOmin o~' , eZi~e " raised to ten minutes, both the characteristic temperatures (ie T,(] O) and Tw(lO)) dropped to about520°F. For the polyester based polymer concrete, after a two minute exposure, no apparent loss of strength was found below T,(2) ~, 740°F and no weight loss was observed below T~(2) ,~, 620°F; after a ten minute exposure, I I II I J I T~(IO) and Tu/IO) dropped to about 600 800 I000 400 610°F and 540°F, respectively. The TEMPERATURE (OF) measure of weight loss is more sensitive than that of structural integrity, not only because weight loss Fig 5 Weight loss of polyester res/n based PC following 2 rain. and 10 in. was measured instrumentally whereexposures to elevated temperature as structural integritywas determined subjectively, but because Tw(t)~ Tf t) in these tests (and thus weight loss andTw(t), for the experimentally perature. Likewise, increasing the was observed before structural de- selected values of t. At temperatures temperature (to a sufficiently high gradation, in general). A comparison above T,(t), both types of PC exhibit value), for a given time of exposure, of the temperatures Ts(t) and T~(t) for degradation behaviour that is both increases weight loss and decreases the two materials shows that the time and temperature dependent. In structural integrity. Finally for a given polyester basedconcrete has a some- both cases, for sufficiently high time and temperature above Ts(t) and what (although not markedly) supedor temperatures, the longer exposure T~(t), the PMMA based concrete will resistance to thermal degradation time (ten minutes vs. two minutes) experience somewhat greater losses (when compared with the PMMA increases the weight loss and de- of weight and structural integrity than based concrete), since the polyester creases the structural integrity of a the polyester based concrete. concrete has higher values of T~(t) sample exposed to a particular tem-

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CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 1 MARCH/APRIL 1987

17

Acknowledgement (6) The experiments reported in this paper were carried out as part of a project sponsored by The Civil Engineering Laboratory, Port Hueneme, CA, USA, and Mission Research Corp., Santa Barbara, CA, USA.

(7)

REFERENCES ( 1) (2)

(3)

(4)

(5)

Consulting Engineer, May 1984 p 51. HEROD S, Concrete, May 1984 pp 4042. 'Polymer concrete patching materials,' BNL-23369, Brookhaven National Laboratory, Upton, NY September 1977. 'Polymer concrete patching materials,' Final report and user's manual, implementation package 77-11, Vols 1 and I1,Brookhaven National Laboratory, Upton, NY April 1977. 'Concrete-polymer materials for highway applications,' Final report, BNL50462 and FHWA-RD-75-86, Brookhaven National Laboratory, Upton, NY,

(8)

(9)

(10)

(I ] )

June 1975. 'Methyl methacrylate polymer-concrete for bomb damage repair,' Phase l, Engineering & Services Laboratory, Air Force Engineering & Services Centre, Tyndall Air Force Base, Florida, March 1979 - May 1980. 'Methyl methacrylate polymer-concrete for bomb damage repair,' Final report, Engineering & Sen'ices Laboratory, Air Force Engineering & Services Centre, Tyndall Air Base, Florida June 1980 September 1981. Zeldin, A, L E Kukacka, and N. Carciello,'Polymer Systems in Geothermal Applications,' J. Applied Polymer Science 23, 3179-92 (1979). 'Polymer concrete pipe for high-temperature corrosive environments,' BNL28715, Brook,haven National Laboratory, Upton, NY. 'A new, novel well-cementing polymerconcrete composite." BNL-29502, Brookhaven National Laboratory, Upton, NY, September 1980. 'The effect of moisture on the physical and durability properties of methyl

(12) (13)

( ] 4)

(15)

(16)

rnethacrylate polymer concrete.' BNL 33420, Brookhaven National Laboratow. Upton, NY, September 1983. Transmission Distribution. April 1984 p12. 'Development of polymer-bonded silica (Polysil) for electrical applications. Final report, EPRI EL-488, ProJect 480 I. Prepared by Westinghouse Research and Development Centre, Pittsburgh. PA, May 1977. 'Further Development of polysil material systems for electrical applications.' Final report, EPRI EL-I093. Project 1203-], Prepared by Lindsey Industries Incorporated. Azusa, CA, May 1979. 'Field evaluation of new outdoor polysi! insulators,' Interim report, EPRI E l 2635, Project 128]-1, Prepared by LJndsey Industries Incorporated, Azusa. CA, November ]982. Kim K, 'The processing, mechanical properties and microfractography o! polymer concrete composite material, Ph.D. Thesis, Dept. of Mech. Eng., University of California, Santa Barbara. November 1986.

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