CN115073875B - Wet-type anti-slip thermoplastic elastomer and preparation method thereof - Google Patents

Abstract

The application relates to the technical field of elastomer production, and particularly discloses a wet-type anti-slip thermoplastic elastomer and a preparation method thereof. The wet type anti-slip thermoplastic elastomer is obtained by extrusion processing of a mixture, wherein the mixture comprises a material A and a material B, and the material A comprises the following components in parts by weight: 80-120 parts of styrene thermoplastic elastomer, 50-180 parts of filling oil, 10-60 parts of polypropylene, 10-90 parts of rubber filler, 5-30 parts of tackifying resin, 0.1-10 parts of peroxide cross-linking agent, 0.1-10 parts of processing aid and 60-100 parts of inorganic filler. According to the application, the mixture is obtained by combining the material A and the material B, and then the wet-type anti-slip thermoplastic elastomer is obtained by extrusion processing of the mixture, so that the retention rate of the mechanical property of the elastomer after wet heat aging is improved, and the wet-type anti-slip property of the elastomer is also improved.

Description

Wet-type anti-slip thermoplastic elastomer and preparation method thereof

Technical Field

The application relates to the technical field of elastomer production, in particular to a wet-type anti-slip thermoplastic elastomer and a preparation method thereof.

Background

The thermoplastic elastomer is a novel high polymer material between rubber and resin, has high elasticity similar to vulcanized rubber, can freely change shape under the heating condition, has good processability, and can be widely applied to various fields such as daily necessities, medical instruments, vehicles and the like. When the thermoplastic elastomer material is applied to the occasions such as shoe materials, ground mats, tool handles, casters, vapor chamber floors and the like, slipping is easy to occur due to the fact that rainwater, sweat and accumulated water are smeared, normal use is affected, certain potential safety hazards exist, and therefore the thermoplastic elastomer with the wet-type anti-slip effect needs to be related to the occasions.

Chinese patent publication No. CN111117145a discloses a wet-type slip-resistant thermoplastic elastomer and a preparation method thereof, the wet-type slip-resistant thermoplastic elastomer comprises the following components in percentage by weight: 20-25% of hydrogenated styrene/butadiene copolymer, 20-50% of petroleum resin, 5-25% of maleic anhydride grafted polyolefin and 10-25% of polypropylene. The wet skid resistant thermoplastic elastomer comprises the following steps: (1) Mixing the hydrogenated styrene/butadiene copolymer and food-grade white oil, and stirring for 8-10 minutes by a high-speed stirrer until the temperature of the material reaches 70 ℃, and continuing stirring for 4 minutes after the temperature reaches to obtain an intermediate A; (2) Uniformly mixing petroleum resin and maleic acid glycoside grafted polyolefin, and granulating by a double-screw extruder to obtain an intermediate B; and thirdly, mixing the intermediate A, the intermediate B, the polypropylene and the calcium carbonate, uniformly stirring, and extruding and granulating by using a double-screw extruder to obtain the wet-type slip-resistant thermoplastic elastomer.

Regarding the above-mentioned related art, the inventors believe that the thermoplastic elastomer in the related art has a wet slip preventing effect, but when the thermoplastic elastomer is exposed to high temperature and high humidity environment for a long time, the high temperature steam may age the thermoplastic elastomer, affecting the mechanical properties of the thermoplastic elastomer.

Disclosure of Invention

In the related art, when a thermoplastic elastomer is in a high-temperature and high-humidity environment for a long time, high-temperature steam easily affects the mechanical properties of the thermoplastic elastomer. To ameliorate this disadvantage, the present application provides a wet skid resistant thermoplastic elastomer and a method of making the same.

In a first aspect, the present application provides a wet skid resistant thermoplastic elastomer, which adopts the following technical scheme:

the wet type anti-slip thermoplastic elastomer is obtained by extrusion processing of a mixture, wherein the mixture comprises a material A and a material B, and the material A comprises the following components in parts by weight: 80-120 parts of styrene thermoplastic elastomer, 50-180 parts of filling oil, 10-60 parts of polypropylene, 10-90 parts of rubber filler, 5-30 parts of tackifying resin, 0.1-10 parts of peroxide cross-linking agent, 0.1-10 parts of processing aid and 60-100 parts of inorganic filler, wherein the components of the rubber filler comprise styrene-butadiene rubber which is not vulcanized, the components of the material B comprise composite filler and acrylic acid, and the composite filler is prepared from crushed products of waste asphalt pavement.

By adopting the technical scheme, compared with the related art, the wet-type anti-slip thermoplastic elastomer is prepared by selecting the mixture obtained by combining the material A and the material B and then extruding the mixture. The composite filler in the material B is a combination of asphaltene and pavement filler, and the waste asphalt pavement is naturally aged in the original service process, so that the carboxyl content in the asphaltene of the composite filler is greatly improved compared with that of the freshly mixed asphalt, and the carboxyl in the asphaltene of the composite filler is mutually associated with acrylic acid through hydrogen bonds. When the material A and the material B are mixed and jointly subjected to extrusion processing, the acrylic acid and the styrene-butadiene rubber in the material A are crosslinked under the action of a peroxide crosslinking agent, so that a composite of the styrene-butadiene rubber, the asphaltene and the pavement filler is formed. The compound not only realizes the vulcanization of the styrene-butadiene rubber, but also introduces asphaltene into the elastomer, thereby improving the mechanical property of the elastomer. The asphaltene introduced by the composite filler is aged in the original service process, so that the softening point of the asphaltene is higher than that of the common fresh asphalt, and further aging of the asphaltene introduced by the composite filler is difficult to occur even in a damp-heat environment, so that the resistance of the elastomer to damp-heat aging is enhanced, and the retention rate of the mechanical property of the elastomer after the damp-heat aging is improved.

Preferably, the material B comprises the following components in parts by weight: 40-60 parts of composite filler and 8-10 parts of acrylic acid.

By adopting the technical scheme, the proportion of the raw materials of the material B is optimized, and the retention rate of the mechanical property of the elastomer after humid heat aging is improved.

Preferably, the material B also comprises 16-20 parts by weight of chitosan.

By adopting the technical scheme, the molecules in the chitosan contain amino, and the amino in the chitosan molecules can be combined with the carboxyl in the composite filler asphaltene and the carboxyl in the acrylic acid to form carboxylate, so that indirect combination between the acrylic acid and the asphaltene is realized, the combination effect between the acrylic acid and the composite filler asphaltene is enhanced, and the mechanical property of the elastomer is improved. In addition, the water resistance of the chitosan can be improved after the chitosan is subjected to damp heat treatment, so that the permeation rate of high-temperature water vapor in a damp heat environment into the elastomer is reduced, and the retention rate of the mechanical property of the elastomer after damp heat aging is improved.

Preferably, the mixture is prepared from a material A and a material B according to the following steps of (4.5-5.5): 1 weight ratio.

By adopting the technical scheme, the proportion of the material A and the material B is optimized, so that the retention rate of the mechanical property of the elastomer after wet heat aging is improved.

Preferably, the composite filler in the material B component is prepared according to the following method:

(1) Uniformly mixing the crushed products of the waste asphalt pavement with the pretreatment liquid to obtain mixed slurry, and standing the mixed slurry for 18-24 hours; in the step, the components of the pretreatment liquid comprise a softening auxiliary agent and a foaming agent, wherein the foaming agent is a chemical foaming agent with the decomposition temperature of 50-70 ℃;

(2) After standing, heating the mixed slurry for 30-60min under the water bath heating condition of 70-80 ℃, then cooling the mixed slurry to 15-25 ℃, and then filtering to remove the liquid phase in the mixed slurry, wherein the rest solid is the composite filler.

By adopting the technical scheme, the crushed products of the waste asphalt pavement are soaked by the pretreatment liquid, and the softening auxiliary agent in the pretreatment liquid softens the asphaltenes in the crushed products of the waste asphalt pavement and carries the foaming agent to permeate into the asphaltenes. After heating in water bath, the foaming agent foams in the asphaltene on the surface of the broken product of the waste asphalt pavement, and after cooling, a porous structure is formed on the surface of the asphaltene, so that the composite filler is obtained. The porous structure generated by the foaming agent in the asphaltene enlarges the surface area of the composite filler, increases the bonding degree among the asphaltene, the acrylic acid and the styrene-butadiene rubber, improves the mechanical property of the elastomer, and is beneficial to improving the retention rate of the mechanical property of the elastomer after humid heat aging.

Preferably, in the crushed product of the waste asphalt pavement, the softening point of the asphalt component is 65-70 ℃.

By adopting the technical scheme, in the service process of asphalt in the asphalt pavement, along with the natural aging, the softening point of the asphalt is gradually increased, and when the softening point of the asphalt in the crushed products of the waste asphalt pavement is between 65 and 70 ℃, the aging degree of the asphalt is enough to resist the damp and hot environment. When the softening point of the asphalt is higher than 70 ℃, the softening effect of the softening auxiliary agent is limited, so that the foaming agent is not beneficial to entering between the asphalt, and the formation of a porous structure in the asphalt is affected.

Preferably, the chemical foaming agent in the pretreatment liquid is azobisisobutyronitrile.

Through adopting above-mentioned technical scheme, azodiisobutyronitrile can decompose and produce nitrogen gas after being heated, and the nitrogen gas can be so that produce the gas pocket in asphaltene when diffusing in asphaltene to foaming effect has been played.

Preferably, the softening aid is formed by mixing xylene and styrene.

By adopting the technical scheme, the free radical generated during the decomposition of the azodiisobutyronitrile can promote the polymerization of the styrene in the softening auxiliary agent, so that the newly generated air holes are supported, the collapse of the air holes is reduced, the bonding degree among the asphaltene, the acrylic acid and the styrene-butadiene rubber is increased, the mechanical property of the elastomer is improved, and the retention rate of the mechanical property of the elastomer after the wet heat aging is improved.

Preferably, the components of the pretreatment liquid further comprise oil shale residues.

By adopting the technical scheme, the oil shale waste and the filling oil have good compatibility, the oil shale waste added into the pretreatment liquid can be adhered to the surface of the composite filler, and the filling oil is adsorbed when the mixture is extruded, so that the lubricating effect on the composite filler is achieved, the dispersion of the composite filler is promoted, the uniformity of the distribution of the composite filler is improved, and the possibility of influencing the mechanical property of the elastomer due to uneven distribution of the composite filler is reduced.

In a second aspect, the application provides a method for preparing a wet-type anti-slip thermoplastic elastomer, which adopts the following technical scheme.

A preparation method of a wet type anti-slip thermoplastic elastomer comprises the following steps:

(1) Uniformly mixing a styrene thermoplastic elastomer, filling oil, polypropylene, rubber filler, tackifying resin, a peroxide crosslinking agent, a processing aid and an inorganic filler to obtain a material A; uniformly mixing the composite filler and acrylic acid to obtain a material B;

(2) And mixing the material A and the material B, uniformly stirring to obtain a mixture, and extruding and granulating the mixture at 160-240 ℃ to obtain the wet-type slip-resistant thermoplastic elastomer.

By adopting the technical scheme, the wet-type anti-slip thermoplastic elastomer is obtained by mixing the material A and the material B to obtain the mixture and extruding and granulating the mixture.

In summary, the application has the following beneficial effects:

1. in the mixture of the application, the acrylic acid is combined with carboxyl in the composite filler asphaltene, and the peroxide cross-linking agent promotes the cross-linking between the acrylic acid and the styrene-butadiene rubber under the temperature condition of extrusion processing, so that a composite of the styrene-butadiene rubber, the asphaltene and the pavement filler is formed, thereby realizing the vulcanization of the styrene-butadiene rubber and introducing the asphaltene into the elastomer, and improving the mechanical property of the elastomer. Asphaltene introduced by the composite filler is aged in the original service process, so that further aging is difficult to occur even in a damp-heat environment, and the retention rate of the mechanical property of the elastomer after damp-heat aging is improved.

2. According to the application, the broken product of the waste asphalt pavement is soaked by the pretreatment liquid, the foaming agent in the pretreatment liquid permeates into the asphalt under the action of the softening auxiliary agent, the porous structure generated by the foaming agent in the asphalt enlarges the surface area of the composite filler, increases the bonding degree among the asphalt, the acrylic acid and the styrene-butadiene rubber, improves the mechanical property of the elastomer, and is beneficial to improving the retention rate of the mechanical property of the elastomer after humid heat aging.

3. According to the method, the material A and the material B are mixed to obtain the mixture, and the mixture is extruded and granulated to obtain the wet-type anti-slip thermoplastic elastomer.

Detailed Description

The present application will be described in further detail with reference to examples, preparations and comparative examples, and the raw materials according to the present application are all commercially available.

Preparation example of composite Filler

The following is an example of preparation 1.

Preparation example 1

In the preparation example, the crushed product of the waste asphalt pavement is prepared by crushing the waste asphalt pavement with the service life of 10 years, the average grain diameter is 860 micrometers, and the asphalt model is 70# matrix asphalt.

In this preparation example, the composite filler was prepared according to the following method: (1) Uniformly mixing 100kg of crushed products of the waste asphalt pavement with 40kg of pretreatment liquid to obtain mixed slurry, and standing the mixed slurry for 20h; in the step, the components of the pretreatment liquid comprise 35kg of softening auxiliary agent and 5kg of foaming agent, wherein the softening auxiliary agent is dimethylbenzene, and the foaming agent is azodiisobutyronitrile;

(2) And after standing, heating the mixed slurry for 45min under the water bath heating condition of 75 ℃, cooling the mixed slurry to 20 ℃, filtering to remove the liquid phase in the mixed slurry, and airing the rest solid to obtain the composite filler.

As shown in Table 1, the preparation examples 1 to 5 were different in the ratio of the raw materials of the pretreatment liquid.

TABLE 1

Sample of	Softening aid/kg	Foaming agent/kg
			Preparation example 1	35	5
Preparation example 2	34	6
			Preparation example 3	33	7
Preparation example 4	32	8
			Preparation example 5	31	9

Preparation example 6

This preparation differs from preparation 5 in that the softening aid consists of 30kg of xylene and 3kg of styrene.

Preparation example 7

This preparation example differs from preparation example 6 in that 2kg of oil shale residue having an average particle size of 40 μm was also included in the pretreatment liquid.

Examples

Examples 1 to 5

The following description will take example 1 as an example.

Example 1

In the embodiment, the filling oil is 26# naphthenic oil, the average molecular weight of polypropylene is 320000, the rubber filler is styrene butadiene rubber which is not vulcanized, the tackifying resin is C5 hydrogenated petroleum resin, the peroxide crosslinking agent is dicumyl peroxide, and the processing aid comprises hindered phenol antioxidant AT-10, calcium stearate and deodorant GRE-807 according to the following weight ratio of 1:3:4, wherein the inorganic filler is silica fume, and the composite filler is the composite filler of preparation example 1.

In this example, a wet skid resistant thermoplastic elastomer was prepared according to the following steps:

(1) Uniformly mixing 80kg of styrene thermoplastic elastomer, 50kg of filling oil, 10kg of polypropylene, 10kg of rubber filler, 5kg of tackifying resin, 0.1kg of peroxide crosslinking agent, 0.1kg of processing aid and 60kg of inorganic filler to obtain a material A; uniformly mixing 40kg of composite filler and 8kg of acrylic acid to obtain a material B;

(2) Mixing the 160kgA material and the 40kgB material, stirring uniformly to obtain a mixture, and then extruding and granulating the mixture at 220 ℃ to obtain the wet-type slip-resistant thermoplastic elastomer.

As shown in Table 2, examples 1 to 5 are different in the proportion of the A material

TABLE 2

Examples 6 to 9

As shown in Table 3, examples 6 to 9 are different from example 3 in the proportion of the material B.

TABLE 3 Table 3

Sample of	Composite filler/kg	Acrylic acid/kg
			Example 3	40	8
Example 6	45	8.5
			Example 7	50	9
Example 8	55	9.5
			Example 9	60	10

Example 10

This example differs from example 7 in that 16kg of chitosan was also included in the B stock.

As shown in Table 4, examples 10-14 differ in that the mix was made up of 40kgB mix and a mix of different weights.

TABLE 4 Table 4

Sample of	Example 10	Example 11	Example 12	Example 13	Example 14
						Dosage of A material/kg	160	180	200	220	240

Examples 15 to 20

Examples 15-20 differ from example 12 in the preparation of the composite filler, as shown in Table 5.

TABLE 5

Sample of	Preparation example of composite Filler
		Example 12	Preparation example 1
Example 15	Preparation example 2
		Example 16	Preparation example 3
Example 17	Preparation example 4
		Example 18	Preparation example 5
Example 19	Preparation example 6
		Example 20	Preparation example 7

Comparative example

Comparative example 1

Reference is made to a wet slip resistant thermoplastic elastomer prepared in example 1 of chinese patent publication No. CN111117145 a.

Comparative example 2

This comparative example differs from example 3 in that the rubber filler in the A-component is vulcanized styrene-butadiene rubber.

Comparative example 3

This comparative example differs from example 3 in that acrylic acid is not included in the composition of the B batch.

Comparative example 4

This comparative example differs from example 3 in that the composite filler in the B-stock component was replaced with the crushed product of granite.

Performance detection test method

1. Moisture and heat resistance

The detection is carried out according to the following steps:

(1) The tensile strength of the elastomer prepared in each of examples and preparations was examined with reference to GB/T528 determination of tensile properties of vulcanized rubber and thermoplastic rubber, and is designated as T1;

(2) Referring to the specification in GB/T15005-1995 method for testing the damp heat aging of vulcanized rubber, the elastomer in the step (1) is placed for 10 days under the environment with the temperature of 70 ℃ and the humidity of 93%, and the tensile strength of the elastomer is again detected and is marked as T2;

(3) The loss rate of tensile strength was calculated according to T1 and T2, and the results are shown in Table 6.

The loss rate is calculated as follows:

TABLE 6

Sample of	Loss rate/%	Sample of	Loss rate/%
				Example 1	6.8	Example 13	5.7
Example 2	6.7	Example 14	5.8
				Example 3	6.5	Example 15	5.5
Example 4	6.6	Example 16	5.4
				Example 5	6.7	Example 17	5.4
Example 6	6.3	Example 18	5.5
				Example 7	6.1	Example 19	5.1
Example 8	6.2	Example 20	4.6
				Example 9	6.4	Comparative example 1	32.1
Example 10	5.7	Comparative example 2	24.1
				Example 11	5.6	Comparative example 3	22.6
Example 12	5.6	Comparative example 4	20.5

2. Anti-skid property

The static friction coefficient of the test piece made of the elastomer of the present application was examined with reference to GB/T13826-2008 wet type (nonmetallic type) friction material, and the examination results are shown in Table 7.

TABLE 7

Sample of	Coefficient of static friction	Sample of	Coefficient of static friction
				Example 1	0.35	Example 4	0.34
Example 2	0.35	Example 5	0.35
				Example 3	0.35	Comparative example 1	0.31

As can be seen in combination with examples 1-5 and comparative example 1 and with Table 6, examples 1-5 each have a lower loss rate than comparative example 1, indicating that the wet skid resistant thermoplastic elastomer of the present application generates relatively less aging in a hot humid environment than the elastomer of comparative example 1.

It can be seen from the combination of example 3 and comparative examples 2 to 4 and the combination of table 6 that when any one of unvulcanized styrene-butadiene rubber, acrylic acid or composite filler is absent from the mix, the wet heat resistance of the elastomer is affected due to the difficulty in forming a composite of styrene-butadiene rubber, asphaltene and road filler, and the loss of tensile strength of the elastomer after the wet heat treatment is increased.

It can be seen from the combination of examples 3 and examples 6 to 9 and Table 6 that the proportion of the material B selected in example 7 is more conducive to improving the resistance of the elastomer to wet heat aging, and reduces the loss rate of the tensile strength of the elastomer after the wet heat aging treatment.

As can be seen from the combination of example 7 and example 10 and the table 6, the loss rate measured in example 10 is lower than that in example 7, which means that the chitosan in the mixture of example 10 enhances the water resistance of the acrylic acid after the wet heat treatment, thereby reducing the permeation rate of high-temperature water vapor in the wet heat environment into the elastomer and reducing the loss rate of the tensile strength of the elastomer after the wet heat aging treatment.

As can be seen from the combination of examples 10-14 and Table 6, the ratio of material A to material B selected in example 12 is more conducive to improving the resistance of the elastomer to wet heat aging, and reduces the loss rate of the tensile strength of the elastomer after the wet heat aging treatment.

It can be seen from the combination of examples 12, 15-18 and Table 6 that the composite filler selected in example 16 is more conducive to improving the resistance of the elastomer to wet heat aging and reducing the loss rate of the tensile strength of the elastomer after the wet heat aging treatment.

As can be seen from the combination of example 16 and example 19 and the combination of table 6, example 19 promotes the polymerization of styrene by azobisisobutyronitrile, thereby reducing the collapse of pores, increasing the bonding degree among asphaltene, acrylic acid and styrene butadiene rubber, and improving the mechanical properties of the elastomer, thus helping to improve the retention rate of the mechanical properties of the elastomer after humid heat aging.

As can be seen by combining example 19 and example 20 with table 6, the oil shale residues have a lubricating effect by adsorbing the filling oil in the extrusion process, promote the dispersion of the composite filler, improve the uniformity of the distribution of the composite filler, reduce the possibility of influencing the mechanical properties of the elastomer due to uneven distribution of the composite filler, and improve the retention rate of the mechanical properties of the elastomer after humid heat aging.

As can be seen from the combination of examples 1 to 5 and comparative example 1 and Table 7, the samples prepared from the elastomers of examples 1 to 5 of the present application have higher static friction coefficients than comparative example 1 in the wet friction test, which shows that the elastomers of the present application have better anti-slip effect than comparative example 1, and are less prone to slip even if they are stained with rainwater, sweat or water, thus reducing the potential safety hazards caused by slip in high humidity environments.

The present embodiment is only for explanation of the present application and is not to be construed as limiting the present application, and modifications to the present embodiment, which may not creatively contribute to the present application as required by those skilled in the art after reading the present specification, are all protected by patent laws within the scope of claims of the present application.

Claims (7)

1. The wet type anti-slip thermoplastic elastomer is characterized in that the wet type anti-slip thermoplastic elastomer is obtained by extrusion processing of a mixture, wherein the mixture is prepared from a material A and a material B according to the following proportion (4.5-5.5): 1, wherein the material A comprises the following components in parts by weight: 80-120 parts of styrene thermoplastic elastomer, 50-180 parts of filling oil, 10-60 parts of polypropylene, 10-90 parts of rubber filler, 5-30 parts of tackifying resin, 0.1-10 parts of peroxide cross-linking agent, 0.1-10 parts of processing aid and 60-100 parts of inorganic filler, wherein the components of the rubber filler comprise styrene butadiene rubber which is not vulcanized, and the material B comprises the following components in parts by weight: 40-60 parts of composite filler and 8-10 parts of acrylic acid, wherein the composite filler is prepared from crushed products of waste asphalt pavement;

the composite filler in the material B component is prepared according to the following method:

(1) Uniformly mixing the crushed products of the waste asphalt pavement with the pretreatment liquid to obtain mixed slurry, and standing the mixed slurry for 18-24 hours; in the step, the components of the pretreatment liquid comprise a softening auxiliary agent and a foaming agent, wherein the foaming agent is a chemical foaming agent with the decomposition temperature of 50-70 ℃;

(2) After standing, heating the mixed slurry for 30-60min under the water bath heating condition of 70-80 ℃, then cooling the mixed slurry to 15-25 ℃, and then filtering to remove the liquid phase in the mixed slurry, wherein the rest solid is the composite filler.

2. The wet skid resistant thermoplastic elastomer according to claim 1, wherein the component of the material B further comprises 16 to 20 parts by weight of chitosan.

3. The wet skid resistant thermoplastic elastomer according to claim 1, wherein the softening point of the asphalt component in the crushed product of the waste asphalt pavement is 65-70 ℃.

4. A wet skid resistant thermoplastic elastomer according to claim 3, wherein the chemical foaming agent in the pretreatment liquid is azobisisobutyronitrile.

5. The wet skid resistant thermoplastic elastomer according to claim 4, wherein the softening aid is formed by mixing xylene and styrene.

6. The wet skid resistant thermoplastic elastomer according to claim 5, wherein the components of the pretreatment liquid further comprise oil shale residues.

7. The method for producing a wet skid resistant thermoplastic elastomer according to any one of claims 1 to 6, comprising the steps of:

(1) Uniformly mixing a styrene thermoplastic elastomer, filling oil, polypropylene, rubber filler, tackifying resin, a peroxide crosslinking agent, a processing aid and an inorganic filler to obtain a material A; uniformly mixing the composite filler and acrylic acid to obtain a material B;

(2) And mixing the material A and the material B, uniformly stirring to obtain a mixture, and extruding and granulating the mixture at 160-240 ℃ to obtain the wet-type slip-resistant thermoplastic elastomer.