[0001] The present invention relates to redispersible powders and a method of making the same.

BACKGROUND OF THE INVENTION

[0002] The following discussion of the background to the invention is intended to facilitate understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was published, known or a part of the common general knowledge in any jurisdiction as at the priority date of the application.

[0003] Natural rubber is widely produced in different countries. Nowadays, natural rubber is commonly used in the automobile industry for making tires and the household industry for making items such as rubber gloves. However, utilization of natural rubber in large-scale industries such as the construction industry is still limited. Even though natural rubber can enhance adhesion bonding properties, improve the flexibility, tensile strength and other qualities of building materials, natural rubber which is in the liquid form, is typically not suitable for use with cement-based building materials. This is because it is difficult to mix the natural rubber into a dry mortar at the factory. Thus, water-redispersible or redispersible powders (RDP) were developed to facilitate the use of natural rubber in the construction industry.

[0004] At present, RDP prepared from synthetic polymers may be used in the construction industry for various purposes. For instance, RDP may be used as an additive in mortars such as tile adhesives, smoothing mortars, repair mortars and thermal insulation mortars.

[0005] There have been efforts to prepare RDP from natural rubber. An example discloses a water-redispersible powder prepared from a modified natural rubber obtained by mixing natural rubber with at least one radical initiator and/or oxidizing agent, and then further mixing and reacting natural rubber with at least one olefinically unsaturated monomer and with at least one radical initiator, and/or by mixing natural rubber with at least one filler.

[0006] In another example of preparing RDP from natural rubber, the structure of the natural rubber may be modified by at least one radical initiator and/or oxidizing agent so that the RDP would be suitable for hydrophobising and/or reducing the water absorption of building material compositions which are mixed with water and cured. It is appreciable that if the structure of the natural rubber is not suitably modified, the redispersibility and film formation ability of the redispersible powder may be negatively affected. Further, while a consequent building material such as mortar is able to achieve increased hydrophobicity and reduced water absorption, the adhesion bonding property was lower than the mortars made using ethylene-vinyl acetate-based powders.

[0007] In yet another example, RDP is prepared from natural rubber by using fresh natural rubber mixed with ammonia for preserving the rubber, polyvinyl alcohol (PVOH) as a polymer colloid, sodium dodecyl sulfate (SDS) as a surfactant, and sodium hydrogen carbonate (NaHC0 ₃ ) as an anti-caking agent in the ratio of PVOH: SDS = 1 : 1. The mixture is then dried into a redispersible powder. The flowability of mortars using the RDP made by this process was tested and it was found that the mortars exhibited inferior flowability compared to the mortars using a redispersible powder prepared from commercially available modified natural rubber. Furthermore, the mortars also contained many air bubbles, thereby leading to a decrease in the compressive strength and other significant properties of mortars.

[0008] In light of the above, there exists a need to develop a redispersible powder and a method of making the same that ameliorates at least one of the disadvantages mentioned above.

SUMMARY OF THE INVENTION

[0009] A technical problem to be solved by the disclosure or present invention is to provide a redispersible powder that is suitable for use in construction applications such as but not limited to an additive to cement, mortars, grout, combinations thereof and/or related or similar construction materials. In particular, the redispersible powder of the present invention can be obtained through a process without the need to chemically modify a structure of a key raw material.

[ooioi Another technical problem to be solved by the disclosure or present invention is to provide a process for producing a wet mixture as an intermediate step to produce a redispersible powder, the wet mixture suitable for a drying process, in particular but not limited to spray drying via a spray dryer.

[0011] In accordance with an aspect of the invention there is a redispersible powder comprising: a natural rubber or a modified natural rubber; and a mixture having a protective colloid, the protective colloid includes a surfactant, a colloid protective agent and a stabilizer; wherein the stabilizer includes a polycarboxylate ether (PCE).

[0012] It is appreciable that various colloid protective agents, such as a polyvinyl alcohol (PVOH), may be used.

[0013] It is appreciable that there comprise various possible ratios of natural rubber: surfactant: polyvinyl alcohol (PVOH): polycarboxylate ether (PCE), such as 10: (0.75 to 2): (0.45 to 4): (0.75 to 6), where the values of PVOH and PCE are expressed in ranges.

[0014] In accordance with another aspect of the invention there is a redispersible powder obtainable by adding a mixture comprising a protective colloid, to a natural rubber or a modified natural rubber to form a wet mixture; and drying the wet mixture; wherein the protective colloid includes a surfactant, a colloid protective agent and a stabilizer, the stabilizer includes a polycarboxylate ether (PCE).

[0015] In accordance with another aspect, there is a method of making a redispersible powder, the method comprising the steps of: adding a mixture comprising a protective colloid to a natural mbber or a modified natural rubber to form a wet mixture; and drying the wet mixture to form the redispersible powder; wherein the protective colloid includes a surfactant, a colloid protective agent and a stabilizer, the stabilizer includes a polycarboxylate ether (PCE). [0016] In accordance with another aspect of the invention there is a use of a redispersible powder as defined herein for making a colloid or a mortar.

[0017] In accordance with another aspect of the invention there is a cement composition comprising a cementitious binder and a redispersible powder as defined herein.

[0018] In accordance with another aspect of the invention there is a colloid comprising a redispersible powder as defined herein and water.

[0019] In accordance with another aspect of the invention there is a method of making a colloid, the method comprising providing a redispersible powder as defined herein and adding water to the redispersible powder as defined herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0021] Figure 1(a) illustrates a redispersible powder in accordance with an embodiment of the present invention; Figure 1(b) illustrates a colloid formed after water has been added to the redispersible powder of Figure 1(a);

[0022] Figure 2 illustrates the stability of the colloid formed (a) using NRP-1 after standing for a period of 10 minutes; (b) using NRP-1 after standing for 60 minutes; (c) using NRP-2 after standing for 10 minutes and (d) using NRP-2 after standing for a period of 60 minutes;

[0023] Figure 3 illustrates the stability of the colloid formed using mNRP after standing for (a) 10 minutes, (b) 30 minutes, (c) 120 minutes and (d) 240 minutes;

[0024] Figure 4 shows the compressive strength results of mortars made using various redispersible powders after 7 days (7d) and 28 days (28d);

[0025] Figure 5 shows the adhesion bonding results of mortars made using various redispersible powders; [0026] Figures 6(a), 6(b), 6(c) and 6(d) (collectively known as“Figure 6”) illustrate scanning electron microscope (SEM) images at different degrees of magnification of a mortar comprising a redispersible powder in accordance with an embodiment of the present invention dispersed in cement, wherein the redispersible powder is inserted between the particles of cement and does not interfere in the hydration reaction of the cement. The degree of magnification for Figures 6(a), 6(b), 6(c) and 6(d) is 500x, lOOOx, 5000x and 10,000x, respectively.

DETAILED DESCRIPTION

[0027] Particular embodiments of the present invention will now be described with reference to the accompanying drawings. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the present invention. Additionally, unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one or ordinary skill in the art to which the present invention belongs. Where possible, the same reference numerals are used throughout the figures for clarity and consistency.

[0028] As used herein, the term“natural rubber” refers broadly to the elastomeric substances obtained from trees and plants such as hevea and guayule plants. It is appreciable that the term‘natural rubber’ includes the unprocessed or raw form (e.g. latex), the solid form and/or liquid form.

[0029] As used herein, the term“natural rubber” may, in addition to the above, refer to thickened rubber, concentrated rubber, deproteinized rubber, a mixture or combination thereof. It is appreciable that the natural rubber may have been subjected to homogenization or creaming.

[0030] As used herein, the term“modified natural rubber” refers to non-fully modified structure based natural rubber. In particular, the term“modified natural rubber” may refer to a compounded rubber in which the molecules of the rubber particle are partially chemical cross-linking without affecting the colloidal stability of the rubber. As such, the structure of the rubber is partially modified. Specifically, a partially crosslinked compound rubber may be a prevulcanised rubber, the same of which can be obtained from conventional prevulcanisations. Nonetheless, in the present invention it is appreciable that the prevulcanised rubber has a similar appearance to that of unvulcanised rubber which includes the original fluidity, distribution of rubber particles size and stability of colloidal characteristic.

[0031] Throughout the description, the term “redispersible powder” may be abbreviated to “RDP” and it is appreciable that the two terms may be used interchangeably.

[0032] As used herein, all ratios, parts and percentages are expressed generally in dry weight basis unless otherwise specified.

[0033] Throughout the specification, unless otherwise indicated to the contrary, the terms“comprising”, “consisting of’, and the like, are to be construed as non- exhaustive, or in other words, as meaning“including, but not limited to”.

[0034] Throughout the specification, unless the context requires otherwise, the word“comprise” or variations such as“comprises” or“comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

[0035] Throughout the specification, unless the context requires otherwise, the word“include” or variations such as“includes” or“including”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

[0036] As used herein, the term“about” typically means +/- 5% of the stated value, more typically +/- 4% of the stated value, more typically +/- 3% of the stated value, more typically +/- 2% of the stated value, even more typically +/- 1% of the stated value, and even more typically +/- 0.5% of the stated value.

[0037] Throughout this disclosure, certain embodiments may be disclosed in a range format. It is appreciable that the description in range format is merely for convenience and brevity and should not be construed as a limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. Ranges are not limited to integers, and can include decimal measurements. This applies regardless of the breadth of the range.

[0038] Other aspects of the invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

[0039] In an aspect, there is provided a redispersible powder comprising a natural rubber or a modified natural rubber and a mixture of protective colloids. The protective colloid may comprise a colloid protective agent, a stabilizer and a surfactant. The mixture may include other types of protective colloids.

[0040] In various embodiments, the redispersible powder may be made from a natural rubber or a modified natural rubber. The natural rubber or the modified natural rubber is able to be utilized for production of redispersible powder without further chemical modification to the structure of the rubber. As such, and advantageously, the redispersible powder of the present invention may enhance the properties of building compounds such as cement or mortars. This is contrasted with known solutions which make use of reagents such as a radical initiator and/or oxidizing agent to chemically modify the structure of the natural rubber.

[0041] In various embodiments, the natural rubber or the modified natural rubber includes a solid content in an amount of about 55% to about 65% by rubber weight, preferably about 60% by rubber weight. The present invention does not only limit to the said prescribed- ranges of the solid content of natural rubber or the modified natural rubber.

[0042] In various embodiments, the colloid protective agent may be one or more selected from the group consisting of PVOH, a water soluble polysaccharide, a protein and a lignin sulfonate.

[0043] In various embodiments, the mixture of protective colloids comprises the PVOH as a colloid protective agent, which minimizes aggregation. In various embodiments, the PVOH comprises a degree of hydrolysis of about 80%mol to about 94%mol, about 86%mol to about 94%mol or about 86%mol to about 89%mol or about

92%mol to about 94%moI. In a preferred embodiment, the PVOH comprises a degree of hydrolysis of about 86%mol to about 89%mol. In various embodiments, the PVOH has a viscosity of less than or equal to 6 milli-Pascal-second (mPa-s), preferably less than or equal to 5 mPa-s, more preferably about 3 mPa-s to about 5 mPa-s. In a preferred embodiment, the PVOH comprises a degree of hydrolysis of about 86%mol to about 94%mol and a viscosity of about 3 mPa-s to about 5 mPa-s.

[0044] In various embodiments, the PVOH has a molecular weight of less than 50,000, about 9,000 to about 40,000, about 9,000 to about 31,000, about 9,000 to about 23,000, about 9,000 to about 10,000, about 13,000 to about 40,000, about 13,000 to about 23,000, about 15,000 to about 31,000, about 31,000 to about 40,000, about 35,000 to about 40,000, preferably about 20,000 to about 35,000, more preferably about 31,000.

[0045] In various embodiments, the amount of PVOH is present in a proportion of at least 4.5 part per hundred (phr). In other words, there is at least 0.45g of PVOH per lOg of dry rubber.

[0046] In various embodiments, the mixture of protective colloid comprises polycarboxylate ether (PCE) that functions as a stabilizer. The use of a stabilizer may result in a higher yield and productivity rate compared to a RDP that does not contain a stabilizer. Furthermore, the PCE may support efficient interaction between PVOH and the surfactant, thereby facilitating preparation of the RDP. Consequently, the production rate and the product yield of the RDP is high.

[0047] In various embodiments, the PCE is water soluble. PCEs may be characterized by their chemical characteristics or the choice of monomer used to synthesize the PCE. Each PCE may have different characteristics, including chain density, chain length and modifications thereof. PCEs are comb polymers having a main chain having carboxyl groups, and side chains having polyether groups, especially based on polyethylene oxide (polyethylene glycol; PEG) and/or polypropylene oxide. The side chains may comprise ether groups, and/or further functional groups, especially ester and amide groups. In various embodiments, the PCE may comprise a low side chain density. In various embodiments, the PCE may comprise a long-branched chain and a crowded short-branched chain. In various embodiments, the PCE is a methallyl ether based polycarboxylate or a terminal alkenyl polycarboxylate ether. In various embodiments, the PCE is selected from the group consisting of methacrylic acid-co-w- methoxy poly(ethylene glycol) methacrylate ester (MPEG), acrylic acid-co-isoprenyl oxy polyethylene glycol) (TPEG or IPEG) and acrylic acid-co-a-methallyl-oc-hydroxy poly(ethylene glycol) (HPEG). In a preferred embodiment, the PCE is HPEG or TPEG/IPEG,

[0048] In various embodiments, the mixture of protective colloid comprises the surfactant. The surfactant is a compound having an amphiphilic nature. The molecular structure of the surfactant includes polarity portions, which may include hydrophobic and hydrophilic parts. The hydrophobic part may be a branched or linear hydrocarbon which may contain one or more aromatic structures. The hydrophilic part is an ionic or strongly polar group. Advantageously, the surfactant is used in the mixture to prevent aggregation of particles as it can reduce the interfacial energy between solid and liquid (or fluid and fluid) interfaces, thereby minimizing aggregation of particles.

[0049] In another embodiment, the surfactant of the present invention is an anionic surfactant. In the anionic surfactant, a hydrophilic part is negatively charged where the hydrophobic part is a linear hydrocarbon chain in the range of C12-C16. Advantageously, the anionic surfactant attracts the positive charge of other molecules and leaves a negative charge on the surface. This may improve the dispersibility of the rubber particles, thereby leading to a more homogeneous drying process.

[0050] In yet another embodiment, an anionic surfactant is selected from the group consisting of sodium dodecyl sulfate, sodium laureth sulfate, sodium myreth sulfate, sodium pareth sulfate, potassium lauryl sulfate and ammonium lauryl sulfate. In a preferred embodiment, the anionic surfactant is SDS. Advantageously, the surfactant may improve the dispersibility of the rubber particles, thereby leading to a more homogeneous drying process.

[0051] In various embodiments, the ratio of the natural rubber or the modified natural rubber: surfactant: PVOH: PCE is 10: (0.75 to 2): (0.45 to 4): (0.75 to 6).

[0052] In various embodiments, the ratio of the solid content of the natural rubber or the modified natural rubber to the mixture of protective colloids is (1.5:1) to (4: 1) by weight.

[0053] Advantageously, the mixture of protective colloids synergistically minimizes aggregation of the natural rubber or the modified natural rubber. The mixture of protective colloids is carefully chosen to match natural rubber or modified natural rubber and the process of the present invention. As such, the redispersible powder of the present invention may be used in at least a pilot scale production, if not full-scale factory production. Notably, the compressive strength and adhesion bonding of mortars comprising a redispersible powder of the present invention were superior to commercially available redispersible powders, such as VAE. In addition, the particles of the dispersed RDP of the present invention could be mixed in the mortar without interfering in the hydration reaction of cement.

[0054] In a preferred embodiment, the mixture of protective colloids consists essentially of a surfactant, PVOH and PCE. In another embodiment, the mixture of protective colloids consists of a surfactant, PVOH and PCE.

[0055] In various embodiments, the ratio of surfactant: PVOH: PCE is in the range of 1 : (0.6 to 2): (1 to 3).

[0056] When in contact with water, the RDP shows good wettability and redispersibility, so that the powder can be largely or even fully redispersed when in contact with water within a short period of time. The redispersible powders have high colloidal stability and do not coagulate when mixed with water. As illustrated in Table 2, the size of the particles of the obtained RDP when redispersed in water may be approximately the size of the natural rubber used to make the RDP. In various embodiments, the particle size of the RJDP when redispersed in water may be about 3 micrometers to about 5 micrometers.

[0057] In various embodiments, the redispersible powder may further comprise an anti-caking agent comprising silicon dioxide. An example of a RDP comprising silicon dioxide is mNRP-Si. Advantageously, mNRP-Si exhibited superior compressive strength, particularly after a prolonged period of time such as 28 days, and superior adhesion bonding. In some embodiments, the anti-caking agent comprising silicon dioxide is selected from the group consisting of silicon dioxide, fly ash, ground- granulated blast furnace slag, fumed silica, pozzolanic material and calcined clay. Advantageously, the anti-caking agent may activate a pozzolan reaction, thereby functioning as a pozzolan material. In various embodiments, when in the presence of water and mixed with cement, the anti-caking agent comprising silicon dioxide may react chemically with calcium hydroxide in the cement and produce a calcium silicate hydrate phase (CSH), which is the phase that enhances the compressive strength of a cement, thereby resulting in a mortar that has a higher compressive strength. More advantageously, the RDP of the present invention was found to be free-flowing and to have good anti-caking properties. Thus, it can be stored for a long period of time without caking. Based on experimental results, the RDP of the present disclosure could be stored for at least 6 months.

[0058] In various embodiments, the redispersible powder may contain further additives such as thickening agents, coalescing agents, preservative agents such as biocides, herbicides, algicides and/or fungicides, anti-foaming agents, anti-oxidants, preservatives such as preservatives against oxide, heat, ozone, light, fatigue and/or hydrolysis.

[0059] In another aspect, there is provided a RDP obtainable by adding a mixture of protective colloids comprising the PVOH, a PCE and a surfactant to a natural rubber or a modified natural rubber to form a wet mixture; and drying the wet mixture via a process such as spray drying to form the redispersible powder.

[0060] In another aspect, there is provided a method of making a redispersible powder, the method comprising adding a mixture of protective colloids comprising the PVOH, a PCE and a surfactant to a natural rubber or a modified natural rubber to form a wet mixture; and drying the wet mixture to form the RDP at a predetermined temperature. The step of drying may be via spray drying or other suitable drying process to form the redispersible powder.

[0061] In various embodiments, the method further comprises the step of adding an anti-caking agent comprising silicon dioxide to the natural rubber or the modified natural rubber.

[0062] In various embodiments, the step of adding PVOH is after the step of adding PCE and the surfactant to the natural rubber or the modified natural rubber.

[0063] In various embodiments, the method further comprises adding water to adjust the viscosity of the wet mixture. The step of adding water may be after the wet mixture is formed.

[0064] In various embodiments, the method further comprises mixing the wet mixture using a mixer prior to the step of drying.

[0065] In various embodiments, the step of drying the wet mixture is carried out using a spray dryer. In various embodiments, the predetermined temperature is not more than 120°C, and in some embodiments, the temperature is about 100°C to about 120°C. It is appreciable that the process of drying may not be carried out if the temperature is more than 120°C.

[0066] It is appreciable that with the above process, the structure of the natural rubber or the modified natural rubber does not need to be chemically modified before forming the wet mixture. As such, radical initiator(s) and/or oxidizing agents and/or chemical agent grafting to modify the structure of the natural rubber or the modified natural rubber are not required. This allows natural rubber or modified natural rubber to be obtained‘off the shelf. As such, the process of the present invention may be adopted easily. Furthermore, this may lead to a cheaper, more economically viable and easier process of making the RDP because less steps are required and less chemical reagents are used. [0067] In another aspect, there is provided a use of a redispersible powder as described above for making a colloid or a building material such as a mortar.

[0068] In another aspect, there is provided a cement composition comprising a cementitious binder and a redispersible powder as described above.

[0069] In various embodiments, the cementitious binder may be a cement selected from the group consisting of Portland cement, Portland-slag cement, Portland-silica fume cement, Portland-pozzolana cement, Portland-burnt shale cement, blast furnace cement, pozzolana cement, composite cement, calcium aluminate cement, blended cement any other well-known constitutions not disclosed above, and one or more combinations thereof. In a preferred embodiment, the cement is Portland cement. In some embodiments, the cement selected from the aforementioned group reduces demand for water.

[0070] The cement composition comprises the cementitious binder in an amount not less than 10 weight percent, not less than 20 weight percent, and/or not less than 30 weight percent.

Studies

[0071] The efficacy and suitability of different materials that may be suitable for the process of obtaining a redispersible powder in accordance with the described embodiments were studied. In addition, certain process parameters and amount of materials were varied.

[0072] In one study, PVOH at different degrees of hydrolysis were used when developing the process of making a redispersible powder of the present invention and their effect on the process of making the RDP was studied. It was found that varying the hydrolysis level of PVOH has an effect on the yield of the RDP due to the effect of the PVOH on the spray drying process and water dispersion ability of the RDP.

[0073] In particular, the inventors discovered that when PVOH having a range of degree of hydrolysis of about 80%mol to about 94%moI, about 86%mol to about 94%mol, about 86%mol to about 89%mol or about 92%mol to about 94%mol was used, fair to very good productivity of spray drying was obtained.

[0074] In another study, the viscosity of the PVOH were varied on the effect on the resultant wet mixture studied. It was found that in generally PVOH that has a viscosity of less than or equal to 6 mPa s, preferably less than or equal to 5 mPa-s, more preferably about 3 mPa-s to about 5 mPa s, mixing with the natural rubber was satisfactory and allowed spray drying to occur efficiently. As such, the viscosity of the PVOH is selected such that there is effective mixing when the PVOH comes into contact with liquid natural rubber and the resultant mixture is suitable for spray drying.

[0075] Similarly, the molecular weight of the PVOH also had an effect on the resultant RDP and process of making the RDP. While a range of molecular weight was investigated, it was found that a molecular weight of less than 100,000 was preferred. If a PVOH having a molecular weight of >100,000 is used, spray drying cannot be carried out. As such, it is preferable that the PVOH has a molecular weight of <100,000, more preferably <50,000.

[0076] In yet another study, different types of superplasticizers were studied. As context, in the construction industry, superplasticizers may be added to reduce the unit water weight of concrete, to produce high strength concrete and a highly flowable concrete. Two categories of superplasticizers were used, namely, polycarboxylate based and polycondensate based superplasticizers, to investigate their effect on the resultant RDP. As shown in Table 1, when polycarboxylate-based superplasticizers were used, particularly MPEG, TPEG /IPEG and HPEG, the resultant RDP exhibited good redispersibility in water. It was found that HPEG and TPEG/IPEG were preferred. Based on these results, it was found that all types of polycarboxylate-based superplasticizers are more suitable with materials such as cement with natural rubber as a co-protective colloid or a stabilizer, thereby providing redispersibility in water and a free-flowing powder. It was found that polycondensate based superplasticizers may not be suitable for the process to produce redispersible powders, in part due to significant aggregation which makes it unsuitable for spray drying.

Table 1 : Different Types of Superplasticizer used as a stabilizer and their Effect

General process of making a RDP

[0077] The process of preparing a redispersible powder (RDP) based on a natural rubber or modified natural rubber comprises the following steps; [0078] Mixing a natural rubber with a mixture of protective colloids comprising the

SDS, the PVOH and the PCE. The ratio of SDS; PVOH: PCE is in the range of 1 : (0.6 to 2): (1 to 3), wherein the mixture of protective colloids is used in an amount of 8 - 30 grams.

[0079] As an optional step, an anti-caking agent comprising silicon dioxide is added, if anti-caking agent is added, the ratio of the anti-caking agent: mixture of protective colloid is 1 : 1 to 1:2,

[0080] Thereafter, the viscosity of the resultant wet mixture is adjusted or varied with water so that the resultant wet mixture would be suitable for spray drying by a spray dryer, it is appreciable that the settings of the spray dryer are adjusted so as to be aligned with the viscosity of the resultant wet mixture. The resultant wet mixture will be dried by the spray dryer at an inlet temperature at 100-120°C, feed pump at a speed of 5-20 ml/minute. It is appreciable that the settings of the spray dryer can be adjusted to another parameter.

[0081] The process outlined above can also be carried out on a modified natural rubber.

Process of making an example RDP having a ratio of SDS: PVOH: PCE of 1: 2: 1

(with 40 g of dried natural rubber-)

[0082] A type of redispersible powder based on a ratio of SDS: PVOH: PCE of 1: 2: 1 was made using the process as disclosed in the described embodiments. As part of the preparation process, a first step includes pre-preparing a sample containing 40 gram of dried natural rubber. It should be understood that even though natural rubber was used to make the RDP, a suitable modified natural rubber may also be used.

[0083] The next step includes the preparing of a solution of surfactant. The surfactant comprises a solution of SDS 7% w/v in 60 ml of water. In addition, a solution of a PVOH in accordance with an embodiment of the present invention was prepared, in this case, a solution of PVOH 13% w/v in 60 ml of water.

[0084] The step of preparing a wet mixture includes the following sub-steps as follows: 1. Adding 4ml of PCE, wherein the PCE is a methallyl ether-based polycarbox late, having a solid content of 50%, into 60ml of the SDS solution and 40 g of dried natural rubber, then mix the two components using a mixer at 300-500 rounds per minute (rpm) for 5 -20 mins;

2. Adding the PVOH solution into the solution of (1) and then mixing the mixture using a mixer at 300 -500 rpm for 5 min prior to spray drying. Advantageously, mixing the mixture in this manner leads to the formation of a homogeneous mixture and minimizes aggregation.

[0085] The step of spray drying may be preceded by a spray dryer setup step, wherein the spray dryer set up step includes adjustment of the settings of a spray dryer, and parameters chosen such that the spray dryer will be operated at a temperature of between l00-l20°C and has a feed rate of 10-20 ml/min.

[0086] The wet mixture is dried using the spray dryer until a RDP is obtained. [0087] It was found that using the process outlined above, particularly when the PCE has been used in the preparation, the yield of the RDP increased to 90% and there was an increase in the production rate, such that the RDP can he produced at a rate of 85 gram/hour. In particular, the yield of RDP increased because there was a decreased loss arising from decreased film formation occurring at and adhering to an inside wail of the spray drier. Advantageously, all the RDP exhibited homogeneous particle size of 1 - 25 micrometers, an average particle size of 8 - 9 micrometer, as measured by a particle size analyzer (PSD).

Study on different RDPs obtained through varying the amount of SDS and PCE [0088] Different RDPs were made using the process of the present invention, as illustrated in Table 2. In particular, the redisperse denoted by the symbols‘NRP-F and ‘NRP-2' do not contain a PCE, while mNRP and mNRP-Si contain a PCE. In addition, an anti -caking agent comprising silicon dioxide was used to make mNRP-Si.

Table 2: Various compositions of RDP comprising different ratio of the mixture of protective colloid and anti-caking agent

[0089] Note; The above-mentioned ratios are calculated based on 10g of natural rubber (dry). The redispersible powder, mNRP, obtained by the process outlined above is illustrated in dry form in Figure 1 (a). After mNRP is dispersed in water, the size of the RDP particles is similar to that of natural rubber. Figure 1(b) illustrates the colloid formed when the RDP is dispersed in water. The size of natural rubber particles is 2.4 micrometers while the size of particles of the obtained RDP is 3-5 micrometers, as measured by a water-based particle size analyzer (PSD). This is to affirm that the obtained RDP based on a natural rubber and made using the process of the present invention has a property similar to that of the original natural rubber.

Study on stability

[0090] The stability of a colloid formed by dispersing the series of RDP shown in Table 2 in water was tested. Indication of stability of a colloid may be obtained by seeing whether a layer of separation is formed between the RDP and water.

[0091] The test results show that when PCE was not used in the process to make a RDP, such as the redispersible powder NRP-1, a layer of separation between the NRP-1 and water was formed after 10 minutes (Figure 2(a)) and the separation became clearer after 60 minutes (Figure 2(b)). Similarly, for the redispersible powder NRP-2, a layer of separation between the NRP-2 and water was formed after 10 minutes (Figure 2(c)) and the separation became clearer after 60 minutes (Figure 2(d)). In contrast, when PCE was used to make a RDP, such as the redispersible powder mNRP, a layer of separation formed only after 120 minutes (Figure 3) and the separation became clearer after 240 minutes. Consequently, it was determined that the presence of PCE led to a higher stability as evidenced by the significantly longer time for a colloid formed using mNRP to separate into a layer. As a surprising technical effect, the standing time could be significantly increased. As such, PCE was exhibited to have advantageous effects to the stability of the colloid, such that the stability of a redispersible powder containing PCE had significantly greater stability that a redispersible powder that does not contain PCE.

Study on compressive strength and adhesion bonding

A series of mortars was prepared using the series of RDP prepared in Table 2, as illustrated in Table 3. A control mortar was prepared by not using any RDP (formula 1), while a comparative mortar was prepared by using concentrated natural rubber (NRL), which is a liquid natural rubber comprising a solid content of 2% only (formula 2). Another comparative mortar was prepared using a commercially available synthetic RDP, VAE, which is an ethylene-vinyl acetate-based powder. Table 3: Determination of Compressive Strength of Various Mortars

The ratio of solid content of the natural rubber to an amount of the protective colloids used in the experiments is (1.5: 1) to (4: 1 ) by weight.

[0093] The compressive strength and adhesion bonding of the mortars in Table 3 are illustrated in Figures 4 and 5. The compressive strength tests were carried out in accordance with the standard BS EN 12190:1999 for‘Determination of compressive strength of repair mortar’, while the adhesion bonding tests were carried out using standard equipment in accordance with the aforementioned standard. It was found that mortars made of NRP-2, mNRP or mNRP-Si showed superior compressive strength to VAE after 7 days or 28 days. Furthermore, mortars made of mNRP or mNRP-Si showed superior compressive strength to the control. This shows that with the use of PCE, the resultant RDP does not interfere with the hydration reaction of the cement. Surprisingly, a compressive strength at day 28 of a mortar of formula 7 comprising a silicon dioxide as an anti-caking agent is significantly higher than the other mortars listed in Table 1. In fact, the increase in compressive strength was significantly greater between mNRP at day 28 as compared to day 7. This shows that the mortar made of mNRP-Si shows superior compressive strength, especially for the late strength of the mortar. The results also showed that the RDP of the present invention (formula 4 - 6) did not negatively affect the compressive strength of the mortar. This is because the particles of the dispersed RDP can be mixed in the mortar without interfering in the hydration reaction of cement. This was confirmed by SEM images of a mortar made of a RDP of the present invention, wherein the RDP is mNRP (Figure 6).

[0094] As shown from the results, it has been found that a mortar comprising PCE- based RDP of the present invention provides an adhesion bonding higher than a mortar comprising NRL and the commercial RDP, VAE. In particular, Figure 4 illustrates that the adhesion bonding of a mortar comprising mNRP or mNRP-Si exhibited a higher adhesion bonding by about 30%. In addition, the adhesion bonding of the mortar comprising mNRP or mNRP-Si exhibited higher adhesion bonding compared to mortars comprising VAE.

[0095] Consequently, the results show that RDPs of the present invention can provide a water-redispersible when used in cement or mortar. Advantageously, mortars made of RDPs of the present invention possessed an increase in compressive strength and adhesion bonding without having to modify the structure of natural rubber.

[0096] It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention. It is intended that all such modifications and adaptations come within the scope of the appended claims.

[0097] Further, it is to be appreciated that features from various embodiment(s), may be combined to form one or more additional embodiments.