The invention relates to a composite material comprising a particulate material comprising metal oxides and a polymer comprising phosphonic acid groups, to a method for producing a composite material, and to products comprising the composite material.

Background Art

One of humankinds first construction materials that is still being used today is mud. Currently, 30% of the world’s population still lives in structures made from mud. Most of these inhabitants are found in developing countries where housing needs to be cheap, built with limited use of technology , and the raw materials need to be easily accessible and available. Costs for this type of construction material are low because the materials utilized are local, and the soil/sand used can be found very close to where construction is going to occur. As a result, there is very little pollution caused by the transport of materials compared to traditional methods like concrete. An example of this type of Earth-based construction material is Adobe. Abode bricks were made with humid sandy soil that once mixed, molded, and dried could be used to build structures. Adobe buildings are known to last hundreds of years and many are still around today. The reason more developed countries may have moved on from Earthbased materials to those like concrete may be due to the increased compressive strength values that materials like concrete allow. For example, the compressive strength of concrete used in residential buildings is generally at least about 15 MPa.

However, if the mechanical properties of Earth-based materials could be improved to provide values that are on par with concrete, these materials may be used as a basis for a wide range of applications, ranging from building materials to electronics. Such Earth-based materials would vastly reduce the environmental impact as the raw material - soil - is abundantly present and eliminates the need for extracting and producing rare and expensive resources. Moreover, as soil is abundantly present at the sites of desired constructions or other desired uses, the need for transportation and the related environmental fuel issues are overcome.

Description of the invention

Therefore, it is an objective of the present invention to provide a material that can be derived from soil, and which has improved mechanical properties compared to traditional soil- derived materials such as adobe. Thereto, the invention provides a composite material comprising a particulate material comprising metal oxides, and a polymer comprising phosphonic acid groups.

Without wishing to be bound by theory, it is believed that when exposed to metal oxide surfaces, phosphonic acid groups can form a chemical bond that acts as an anchor, through a condensation reaction with the subsequent release of water. Since phosphonic acids are diacids they can have more than one potential binding site when exposed to metal oxide surfaces. This allows for the use of phosphonic acid functionalized small molecules to create thin layers on surfaces which enables the ability to tune and modify the properties of the surface. In the prior art, this has been used for advancements in fields like engineering and electronics to create protective coatings, as well as biological and analytical sensors.

It has now surprisingly been found that when a solution comprising a polymer comprising phosphonic acid groups is blended with a particulate material comprising metal oxides, such as sand/soil samples, and cured, a composite material is provided having compressive strength values on par with concrete. It has also been surprisingly found that the composite material may absorb water at ambient conditions to become workable/reformable. After reforming the material, the workable material can be cured again to become solid, with identical compressive strength values as before the reforming.

The invention also provides a method for producing a composite material, the method comprising: a) providing a solution of a polymer comprising phosphonic acid groups in a solvent, b) mixing the polymer solution with the particulate material comprising metal oxides thereby providing a composite mixture, and c) curing the composite mixture.

Curing as used in the invention can be performed, for example, by removal of solvent. Preferably, the solvent is water. Curing also involves removal of any additional water which may be present in the composite material. Additional water may be the result of a dehydration reaction involving the metal oxides and the polymer, and/or additional water may have been added to the material to make it workable. Preferably, curing results in the polymer being ionically and/or covalently bonded to the metal oxides accompanied by release of water. Curing may be performed at ambient conditions. In this case, solvent and/or water is allowed to evaporate from the composite mixture. Especially for larger structures which do not fit in an oven, this is in advantage as no special measures need to be taken. Alternatively, curing may be conducted by heating the composite mixture, such as heating the mixture to a temperature between 50 °C and 200 °C, preferably between 100 °C and 180 °C, such as about 150 °C in order to remove solvent and/or water from the composite mixture. Preferably, curing is performed under low relative humidity, such as lower than 35% RH. Preferably, curing is performed for at least 1 hour, such as at least 2 hours or 3 hours. More preferably 24 hours to ensure maximal curing.

The invention further provides a product comprising a composite material according to the invention as well as a product comprising a composite material obtained by the method according to the invention.

Besides use as a composite construction material, the composite materials of the invention may be used in many more applications where it is particularly advantageous that the composite materials absorb water at ambient conditions to become workable/reformable .

For example, the composite materials may be used in healthcare, as a cast to fixate broken bones. The cast may be easily removed again upon softening the material with water. Thus, the invention provides a cast comprising the composite material. A dental mold comprising the composite material is also envisaged.

The composite material may serve as a clay-like toy. The composite material may be shaped, solidified, and reformed upon softening the material with water. Thus, the invention provides a toy comprising the composite material.

The invention further provides a mold comprising the composite material, such as a normal mold or a reverse mold. Preferably, the mold is a single-use mold. Such a single-use mold can be removed by addition of water to the mold.

The invention further provides a construction element comprising the inventive composite material. The construction element may be a wall, floor, or roof. The construction element may be a brick. The construction element may be an underlayer for roads. It may further be a fixation element. Such a fixation element may for example be used for fixing a pole in place, after which it may be removed upon addition of water. The invention further provides a building, such as a seasonal building comprising the composite material and/or the construction element. A seasonal building may easily be demolished again after use.

The invention further provides for a packaging material comprising the composite material.

The invention further provides for a coating covering electronics and/or electronics housing comprising the composite material.

The invention further provides for a single use item comprising the composite material. Such an item may be comparable to a single-use plastic The invention further provides a removable scaffold comprising the composite material. For example, a scaffold for building a structure that is to be emersed underwater.

The invention further provides for a 3D printed article comprising the composite material.

The invention further provides for furniture comprising the composite material.

The invention further provides for a sculpture comprising the composite material.

The invention further provides for a plant container comprising the composite material.

The invention further provides for a flexible buoy or dock comprising the composite material.

The invention further provides for a barrier, preferably a noise barrier, comprising the composite material.

The invention further provides for a decorative surface, such as a countertop or flooring, comprising the composite material.

The invention further provides for emergency housing comprising the composite material.

The invention further provides for a glue comprising the composite material.

In fact, in the distant future, it may be foreseen that structures may be built on the moon or other locations away from the planet earth, using the polymer and any available soil on the location.

The invention further provides for use of the composite material for repairing damaged concrete or asphalt surfaces.

The invention further provides for use of the composite material for stabilizing soil, for example in areas prone to landslides or erosion.

The invention further provides for use of the composite material for additive manufacturing/3D printing of an object.

The invention further provides for use of the composite material for creating flexible barriers or seals for containing or controlling environmental pollutants, such as oil spills.

The invention further provides for use of the composite material for sealing underground wells or pipes.

The invention further provides for use of the composite material for coating oil wells. For example, the material can be used to prevent water or other elements from the earth formation from seeping into the oil well.

The invention further relates to a method for producing a composite material, the method comprising: a) providing a solution of a polymer comprising phosphonic acid groups in a solvent, b) mixing the polymer solution with a particulate material comprising metal oxides, c) curing the composite mixture, characterized in that the composite material becomes workable upon addition of water, preferably under ambient conditions.

The invention therefore also relates to a method of at least partially reversing curing of a cured composite material comprising a particulate material comprising metal oxides and a polymer comprising phosphonic acid groups, preferably by contacting the material with water, for allowing the material to be reshaped or recycled. It will be understood by the skilled person that water may be in the liquid or gas phase.

Description of the figures

Fig. 1. Embodiment of the invention with a PVPA-co-AA copolymer, cured to a metal oxide with release of water. The particle is characterized “dirt”, but may be any particulate material as described herein.

Fig. 2. Compressive strength of cured mixtures of sand with a solution of PVPA-co-AA copolymer comprising 30% of PVPA, with and without (w/o) chain transfer agent (CTA).

Fig. 3. Compressive strength of cured mixtures of sand with a solution of PVPA-co-AA copolymer comprising 10% of PVPA, with and without (w/o) chain transfer agent (CTA).

Fig. 4. Compressive strength of cured mixtures of sand with a solution of PVPA-co-AA copolymer comprising different percentages of PVPA (with CTA). In each case 3 mL of polymer solution was added.

Fig. 5. Influence of curing time and conditions on strength of a mixture of sand (43 g) with PVPA-co-AA copolymer (10:90 VPA:PAA, 3 mL).

Fig. 6. Effect of Tartaric acid on strength of a mixture of sand (43 g) with PVPA-co-AA copolymer (10:90 VPA:AA, 3 mL).

Fig. 7. Effect of EDTA and disodium EDTA on strength of a mixture of sand (43 g) with PVPA-co-AA copolymer (10:90 PVPA:AA, 3 mL).

Detailed description of the invention

One of the advantages of the invention, is that the composite material may be produced on-site, such as at a building site. Materials found on site are generally not single component materials (for example a sand comprising mixed grains of several types of minerals). It has been shown by the present inventors that such materials are very suitable for creating the composite material. Preferably, the particulate material comprises a further component. In other words, the particulate material preferably is not a single component material. Preferably the particulate material comprises particles having a size in the range of 0.0002 mm to 2 mm, more preferably in the range of 0.002 to 2.0 mm, most preferably 0.063 mm to 2.0 mm. Preferably, at least 50 wt%, such as at least 70 wt%, at least 90 wt% or at least 99 wt% of the particles have the defined size.

Preferably the particulate material comprises soil particles having a size in the range of 0.0002 mm to 2 mm. Most soil is composed of varying amounts of sand, silt, and clay. Sand, silt and clay are particulate materials composed of finely divided mineral particles. All three comprise metal oxides but differ through their size and crystal structures. ISO 14688- 1 :2017 defines sand as particles having a size in the range of 0.063 mm to 2.0 mm, silt as particles having a size in the range of 0.002 to 0.063 mm, and clay as particles having a size smaller than 0.002 mm. Preferably, clay particles are larger than 0.0002 mm. Preferably, the particulate material is soil derived, i.e. extracted from soil by removing the organic material from the soil. Preferably, the particulate material comprises or is sand, silt, and/or clay. More preferably the particulate material is sand or silt. Most preferably, the particulate material is sand.

The particulate material, such as the sand, silt and/or clay may comprise a variety of minerals. Preferably, the particulate material further comprises a non-metal oxide mineral, preferably a silicon dioxide mineral such as quartz. Soil-derived sand, silt, and/or clay comprises relatively high amounts of silicon dioxide. Preferably, the particulate material comprises at least 0.1 wt%, such as at least 1 wt%, or at least 10 wt% of silicon dioxide.

Mineral sand, silt and/or clay comprise relatively high amounts of heavy minerals. The particulate material may comprise or be a mineral sand. The particulate material may also comprise or be an ore, such as iron ore.

Alternatively, recycled resources are also envisaged as the particulate material. For example, crushed glass, wherein the size is reduced to the abovementioned preferred size ranges may be used as the particulate material. Artificially produced materials with a composition resembling soil are also envisaged as the particulate material. A particularly preferred particulate materials is mineral sand. Mineral sands are a class of ore deposit, and may comprise zirconium, titanium, thorium, tungsten, and rare-earth elements. Thus, preferably the particulate material comprises any one or more of zirconium, titanium, thorium, tungsten, and rare-earth elements.

In some cases it may be preferred to use pure particulate materials. For example, it may be preferred to use pure metal oxide particulate material, for example aluminum oxide or zinc oxide. The composite material may further comprise a functional material, such as a conductive material. In the method of the invention, the functional material may thereto be mixed with the polymer solution and the particulate material comprising metal oxides in step b). This results in a functional material, such as a conductive composite material.

Preferably, the composite material is a composite construction material. Polymer composite materials made strictly by mixing a polymer solution with sand/soil have not been explored as a potential replacement for concrete in construction. Given that the composite material has compressive strength values on par with concrete, the composite material is very well suitable as a construction material. With the environmental impact of the production and transportation of construction and building materials, and the related ever-increasing costs, there is a need for alternative methods of construction where the bulk of materials can be found on-site, and structures can be erected quickly. Thus, the invention also provides for use of a polymer comprising phosphonic acid groups for preparing a construction material.

As mentioned hereinbefore, the composite material of the invention may (re-)absorb water under ambient conditions, by adding water or otherwise when the material is in contact with water. It will be recognized by the skilled person that absorption of water by a construction material to be used outdoor may be undesirable. Hence, the invention also provides for materials that do not absorb water. For example, the material can be coated to prevent water/humidity effects, more specifically to prevent water re-absorption.

Preferably, the composite material is obtained by curing a mixture comprising the particulate material and the polymer. Thus, preferably the composite material is cured. The cured material does not release any water upon heating the material to a temperature of 100 °C or higher.

Preferably, the polymer is a copolymer, more preferably with a molecular weight of at least 50,000 g/mol. Polymers comprising phosphonic acid groups are generally expensive, and difficult to polymerize to high molecular weights, such as molecular weights higher than 50,000 g/mol, or molecular weights higher than 100,000 g/mol. For example, research conducted by Bingol et al. regarding the free-radical polymerization of vinyl phosphonic acid (VPA) showed that PVPA is synthesized through a cyclopolymerization mechanism which starts with VPA anhydride, and this slows down the rate of reaction. As a result, the production of high molecular weight PVPA is difficult when the concentration of monomer is high. Copolymerization has been found to result in materials with higher molecular weights. The inventors of the present invention have found that the use of copolymers comprising phosphonic acid groups result in materials with outstanding mechanical properties. This may partly be caused by the higher molecular weights of the obtained copolymers. Unexpectedly, linearly increasing the amount of phosphonic acid groups in the copolymer did not result in a linear increase of the mechanical properties of the resulting material. Thus, copolymers comprising phosphonic acid groups may be used to result in a material in which the cost and performance are optimized. Moreover, the used polymers may be functionalized with crosslinkers.

More preferably, the polymer is a polyvinyl phosphonic acid polymer or copolymer, as polyvinyl phosphonic acid is the most basic phosphonic acid group-comprising polymer, and therefore the raw materials are most abundantly available. In case of a copolymer, the polymer may for example comprise carboxylic acid groups. Even more preferably, the polymer is a copolymer comprising phosphonic acid and carboxylic acid groups. The polymer comprising carboxylic acid groups may for example be polyacrylic acid or polymethacrylic acid.

Most preferably, the polymer is a copolymer of vinyl phosphonic acid and acrylic acid (PVPA-co-PAA). Acrylic acid (AA) is cheaper than VPA and getting high molecular weight poly acrylic acid (PAA) is not as difficult. Therefore, a balance between VPA:AA molar ratios that provided strength with corresponding high molecular weights was found. Thereto, preferably, the copolymer comprises from 1 - 30 mol% of vinyl phosphonic acid (VPA), based on the total molecular weight of the polymer. More preferably, the copolymer comprises from 1 - 20 mol% of VPA, more preferably from 1 - 10 mol% of VPA, yet more preferably from 1 - 2 mol%, most preferably about 2 mol%.

Preferably, the polymer of the invention has a number average molecular weight M _n in the range of 50,000 g/mol to 2,000,000 g/mol, preferably from 50,000 to 1,000,000 g/mol, most preferably from 100,000 to 500,000 g/mol. The number average molecular weight M _n may be determined by gel permeation chromatography (GPC) of a methylated version of the polymer in THF using polystyrene calibration standards.

A detailed method for determining the number-average (M _n ) and weight-average (M _w ) molecular weights by GPC comprises: using a Viscotek GPCmax VE2001 at 35 °C equipped with a 305 TDA (triple detector array: DRI, LS, Vise.) and PAS-103, PAS-104, and PAS-105 Styrene-Divinylbenzene gel columns; fixing the flow rate at 1.0 mL/min using tetra hydrofuran (THF) as the eluent; preparing all GPC samples nominally at 1 mg/mL in THF and filtering through a 0.22 pM PTFE filter into a 1 mL chromatography vial.

Preferably, the composite material comprises from 0.5 - 10 wt% , preferably from 1 - 5 wt%, most preferably from 2 - 5 wt% of the polymer based on the weight of the composite material.

Preferably, the composite material has an unconfined compressive strength at a constant strain rate of 6 mm/sec of > 2 MPa, preferably > 10 MPa, more preferably > 15 MPa, most preferably > 20 MPa.

Preferably, the method of the invention further comprises addition of an a chelating agent in step b). It was found that the presence of a chelating agent in the composite mixture reduces the curing time. Examples of chelating agents are ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), n-hydroxyethylethylenediaminetriacetic acid (HEDTA), as well as simple organic acids like oxalic acid, malic acid, tartaric acid, rubeanic acid and citric acid. More preferably, the chelating agent is chosen from the group consisting of tartaric acid, EDTA, salts thereof and mixtures thereof. Preferably, the chelating agent is added in an amount of 0.1 - 10 wt%, more preferably 1 - 10 wt%, most preferably 5 - 10 wt%, based on the weight of the particulate material.

Preferably, the polymer solution of step a) of the method of the invention is prepared by (co)polymerizing monomers in the solvent, preferably without further purification. This provides for a very efficient method, eliminating the need for expensive and time consuming preparation steps, such as precipitation, purification, and redissolving of the polymer. Polymerization may for example be performed by gradually or step-wise combining one or more separate solutions of the monomers in the solvent with a further solution comprising an initiator and optional heating, e.g. to a temperature between 50 °C and 100 °C. A suitable initiator is for example 2,2’-azobis(2-amidinopropane) dihydrochloride (AAPH). A suitable amount of initiator is about 0.01 mol% to about 1 mol%, preferably from 0.05 mol% - 0.5 mol%, such as about 0.1 mol%, based on the total amount of monomer.

Preferably, polymerization is performed with addition of a chain transfer agent. Use of a chain transfer agent reduces the molecular weight and thereby the viscosity of the resulting polymer. Surprisingly, the use of a chain transfer agent resulted in materials with higher compressive strengths, especially for copolymers comprising 10 mol% of VPA or less. More preferably the chain transfer agent is 1 -octanethiol. Preferably, the amount of chain transfer agent is about 0.5 mol%, based on the total molecular weight of the monomers. Thus, an optimal molecular weight is neither too high, nor too low. The abovementioned preferred molecular weights are optimal.

Preferably, the polymers of the invention are random copolymers. Nevertheless, given the difference in reactivity ratios the resulting polymers may have a non-random distribution of monomers.

Preferably, the method of the invention further comprises at least partially reversing the curing of the material. This allows the material to be reshaped or recycled. Curing may for example be reversed by adding water to the material, or vice versa by adding the material to water, i.e. by contacting the material with water. Preferably, reversing the curing of the material is performed under ambient conditions (i.e. 1 atm and 20 °C). Thus, preferably, the composite material becomes workable upon addition of water, more preferably upon addition of water under ambient conditions. After reversing the curing, the material can subsequently be re-shaped and re-cured.

Preferably, the method further comprises separating the particulate material from the polymer. This enables recycling of the components. Upon contact with a sufficient amount of water, the curing may be reversed, thereby resulting in a mixture of the individual components in water. Such a mixture may be separated, for example by sieving, to obtain the particulate material and a solution of the polymer. The polymer may be obtained from such a solution by evaporating the solvent, and optional purification of the polymer.

As indicated hereinbefore, the composite material of the invention may absorb water to become workable/reformable. Although this is seen as an advantage in many applications, it may be undesirable, for example if the material is to remain its optimal strength under humid or moist conditions. Thereto, the polymer in the composite material is preferably crosslinked. Thus, preferably, the method of the invention further comprises crosslinking of the polymer. Crosslinks are preferably chemical crosslinks (as opposed to physical crosslinks, which may be more easily reversible). Chemical crosslinking may for example be achieved by addition of a crosslinker, and/or by photochemically induced crosslinking, such as by illumination with UV-light. Preferably the UV-light has a wavelength of about 100 - 400 nm.

Additionally or alternatively, the composite material comprises a water repellent and/or water proof coating. Such a coating may for example be applied by applying the coating onto the material or by immersing the composite material in the coating. The coating prevents water from entering the material, thereby making it possible to use the composite material under humid and/or wet conditions. Suitable for the purpose are for example silane and siloxane coatings commonly used for sealing brick houses.

Materials and methods Materials

All chemicals were used without further purification unless otherwise stated. Vinylphosphonic acid (VPA) (>95%) and 1 -Octanethiol (>98%) was purchased from TCI Ltd. Acrylic acid (AA) (99%) and 2,2'-Azobis(2-methylpropionamidine) dihydrochloride (AAPH) (97%) were purchased from Sigma-Aldrich Ltd. S25-3 sand from Sigma Aldrich (acid washed, typically used to pack chromatography columns) was used as the sand.

Synthesis of PVPA-co-AA

The following method details the synthesis of the 10:90 PVPA-co-AA polymer solution. A variety of polymer solutions with different ratios were produced, these are summarized in table 1. Some reactions had 1 -octanethiol a chain transfer agent (CTA) added to control the polymer molar masses. For the homopolymerization of AA and VPA, 1 -octanethiol and the initiator, AAPH, were added at the beginning of the reaction with the diluted monomer, this was allowed to polymerize for 3 hours. VPA (2.57 g, 23.8 mmol) was dissolved in deionized water (7.5 mL) and added to a dry 100 mL round bottom flask equipped with a stirrer that been purged with argon gas. The solution was heated in an oil bath to 90°C and left to stir for 30 minutes. AA (15.45 g, 214.4 mmol) was dissolved in deionized water (8 mL), AAPH (64.5 mg, 0.24 mmol) was dissolved in deionized water (8 mL), and 1 -octanethiol (174.5 mg, 1.19 mmol) was dissolved in deionized water (10 mL).

These were added to the reaction vessel in equal portions, every 30 minutes, for 6 hours. After the last addition the solution was left to stir at 90°C for 18 hours. The resulting polymer solutions comprised about 30 - 40 % (w/v) of the polymer. Unless otherwise stated the polymer solution was not purified prior to preparing the polymer composite samples.

Table 1 : Composition of Polymers and Molecular weights

Characterization

NMR

1H-NMR spectra were recorded on a Bruker AVANCE300 (300 MHz) 5 or Bruker AC300 (300 MHz) 5 NMR spectrometer. ³¹ P-NMR spectra were broadband decoupled and recorded on a Bruker AC300 (121.4 MHz) 5 NMR spectrometers with phosphoric acid in water as the external reference. The following abbreviations are used for NMR peak multiplicities: s, singlet; t, triplet; dd, doublet of doublets; m, multiplet; br. , broad. Chemical shifts are reported in parts per million (ppm) relative to water at (5 4.79) for ¹ H-NMR and phosphoric acid in water (5 0.00) for ³¹ P-NMR.

Gel Permeation Chromatography

Number-average (M _n ) and weight-average (M _w ) molecular weights were determined by size exclusion chromatography using a Viscotek GPCmax VE2001 at 35 °C equipped with a VE 3580 Rl detector and two PAS-104 Styrene-Divinylbenzene gel columns. The flow rate was fixed at 1.0 mL/min using tetrahydrofuran (THF) as the eluent. All molecular weights are relative to a polystyrene calibration curve. All GPC samples were prepared nominally at 2 mg/mL in THF and filtered through a 0.22 pM PTFE filter into a 1 mL chromatography vial. The samples were methylated before performing GPC. Synthesis of Methylated PVPA-co-AA

PVPA-co-AA copolymer was dissolved in a 1:1 v/v THF/methanol solution for a concentration of 5 mg mL ^-1 . The yellow solution of TMS-diazomethane was added dropwise, which resulted in nitrogen bubbling and discoloration. After approximately 2 mL of the methylation agent was added the reaction mixture held its yellow color and was allowed to stir overnight. The next day the reaction mixture was still slightly yellow, although the volume of liquid had decreased. A stream of air was used to blow off the remaining solvent and the reacted polymer was left in the reaction vessel. ¹ H NMR (300 MHz, CDCh) 3.64 (OMe), 2.27 (CH), 2.03-1.65 (CH ₂ ). ³¹ P NMR (121.4 MHz, CDCh) 34.86.

Preparation of composite samples

To prepare the composite samples and uniformly test their compressive strength and curing time, the amount of sand (g) and volume of PVPA-co-AA polymer solution (mL) were monitored and kept consistent. Sand (approximately 43 g) was premeasured in the molds for a certain height and weight, the polymer solution was then added to the sand in a separate mixing glass and mixed manually. Proper care was taken to ensure that a homogenous mixture was achieved. Differences in viscosity between each ratio mix of PVPA-co-AA polymer solution caused some difficulty in mixing due to the stickiness of the polymer but generally the composite materials mixed well. Once thoroughly mixed, the mixture was scooped into cylindrical molds. The sand polymer composites were then compacted so that each sample was of uniform height. The samples were then left to air dry in the fume hood for 4-7 days. When the samples were dry enough to be extracted from the mold without damaging them, they were removed using a Weber press as the samples stuck to the inside of the plastic molds. Once the samples were extracted from the molds, they were baked in an oven for 4 hours at 150°C, this was done to ensure all the water had completely evaporated. The samples were then stored in a desiccator until they could be tested on the compression instrument.

Unconfined compression test

Unconfined compression tests were conducted using a universal mechanical testing system (MTS Model 43) on cylindrical samples with a diameter of about 27 mm and a height of about 51 mm. The applied axial strain throughout the test was constant and maintained at a rate of 6 mm/sec. The test was terminated when the stress exceeded the strength of the composite material. The unconfined compressive strength (UCS) of the samples were determined from the stress-strain graphs resulting from the test, wherein the UCS was the maximum stress value. Setting time measurements

The setting time of each type of PVPA-co-AA polymer composite material was monitored using a Shore durometer. A shore durometer is a device that is used to measure the hardness of a material, typically polymers and rubbers. The durometer measures the depth of an indentation that is allowed by the material given a standardized force by the durometer tip. Two types of durometers were used, a type A and type D. Type A and the type A scale is used for softer materials while type D and the type D scale is used for harder materials. The setting time was tested at consistent time intervals over the course of a few hours to measure the influence of certain variables on setting time. The measurements were terminated if the surface of the sample looked damaged after multiple rounds of testing.

Recycled glass as particulate material

Another interesting aggregate that is sometimes used in the production of concrete is recycled glass. Crushed recycled glass used as sandblasting media was used as a substrate. The composite samples were produced using a 10:90 and 30:70 PVPA-co-AA (with CTA) polymer. The compressive strengths of the samples were 12.1 (2.94%) and 9.2 (3.16%) MPa, for the 10:90 and 30:70 polymers, respectively. Thus, depending on the required compressive strength the polymer glass composites may be useful.

Reshape procedure

1 gram of a PVPA-co-AA polymer comprising 2% of PVPA was dissolved in 3 mL of water. This solution was mixed with 43 g of sand. This resulted in a dough-like material. The sand/polymer mix was manipulated into a shape. The shape was allowed to cure in an oven for 4 hours at 150 °C. The hardened material was removed from the oven and allowed to cool. At this point the material was very hard and strong.

To reuse the material and form a different shape the hardened/cured material was placed in a glass dish. 10 mL of water was poured over the material and the material was allowed to soak in the water for several hours. The material softened and became workable over these hours. Once the material was sufficiently workable the material was reshaped/molded into a different shape. The object was allowed to cure in an oven for 4 hours at 150 °C. At this point the material was very hard and strong.

Results

As can be seen from figures 2 and 3, increasing the amount of polymer in the composite material increases the compressive strength of the material. For copolymers comprising 10 mol% of VPA or less, the addition of a chain transfer agent results in stronger materials. With addition of chain transfer agent, the solutions were also less viscous and easier to handle, i.e. easier to mix with the sand.

As can be seen from figure 4 the compressive strength of cured mixtures of sand with a solution of PVPA-co-AA copolymer comprising different percentages of PVPA are all within the same range and about 20 MPa. Although the samples with 10% of PVPA are the strongest, samples with low PVPA perform well, and are generally more cost effective.

The results in figure 5 indicate that curing is to be performed under conditions that allow removal of water from the composition. Heating the composition during curing particularly increases the curing rate.

Figure 6 indicates the beneficial effect of addition of tartaric acid on strength of a mixture of sand (43 g) with PVPA-co-AA copolymer (10:90 VPA:AA, 3 mL). Already at 5 w% (based on the weight of the sand), tartaric acid was effective. Citric acid was also tested, but needed to be added amounts higher than 5% in order to be effective. On the other hand, additives such as HCI and H2SO4 did not result in any beneficial effect. Also addition of a base, such as NaOH, KOH, or Ca(OH)2 did not result in any beneficial effect either. EDTA, and especially disodium EDTA (see Fig. 7) were beneficial.

The results of the reshape procedure indicate that the material becomes workable upon contact with water, which may advantageously be used in a variety of applications.

Comparing the compressive strength of a 10:90 VPA:AA sample (3 mL) to that of M35 and M40 grade concrete indicated that the compressive strength of the composite material was on par with M40 grade concrete. The material of the invention outperformed the M35 grade concrete.

Comparative experiments were also performed using pure PAA as the polymer, while keeping the other relevant experimental details the same. The compressive strengths of the composite materials according to the invention outperformed those of compositions with pure PAA, especially at concentrations of 1 wt% or higher. The effect was particularly notable for concentrations of 2 wt% or higher.