Process for crosslinking an ethylene-based polymer

Abstract

A method for crosslinking a copolymer of ethylene and at least one other monomer using an organic peroxide represented by Formula (1): wherein R1 is either a methyl or an ethyl group and wherein (i) R1 is a methyl group, R2 is an alkyl group having 1 to 8 carbon atoms optionally substituted with groups containing O, Si, P, S, SO or SO.sub.2 functionalities; (ii) if R1 is an ethyl group, R2 is an alkyl group having 2 to 8 carbon atoms optionally substituted with groups containing O, Si, P, S, SO or SO.sub.2 functionalities. ##STR00001##

Claims

1. A method for crosslinking a copolymer of ethylene and at least one other monomer, the method comprising heating the copolymer at 80-300 C. in the presence of an organic peroxide, wherein the organic peroxide is tert-butyl-1,1-dimethylbutyl peroxide, tert-butyl-1,1,3,3-tetramethylbutyl peroxide, 1,1-dimethylpropyl tert-1,1,3,3-tetramethylbutyl peroxide, and combinations thereof.

2. The method according to claim 1, wherein the copolymer of ethylene and at least one other monomer is an ethylene-vinyl acetate copolymer.

3. The method according to claim 1 wherein the organic peroxide is tert-butyl-1,1,3,3-tetramethylbutyl peroxide or 1,1-dimethylpropyl tert-1,1,3,3-tetramethylbutyl peroxide.

Description

EXAMPLE 1

(1) The organic peroxide used was tert-butyl-1,1,3,3-tetramethylbutyl peroxide (abbreviation: BOP). In each case, 1.1 parts by weight of BOP were added to 100 parts by weight of EVA resin (vinyl acetate content: 32 parts by weight), and uniformly dispersed using two rolls at 65 C. The crosslink characteristics of the resulting EVA resin compound were measured at 160 C. using a curelastometer (JSR Model III). T10 represents the time until the torque reaches 10% of the maximum torque, and T90 represents the time until the torque reaches 90% of the maximum torque. T90-T10 is used as a measure of crosslinking rate. The crosslink characteristics in the process of sealing a solar battery can be estimated from these values. Table 1 shows the results of the test.

(2) Examination of the Occurrence of Voids

(3) BOP (1.1 parts by weight) was added to 100 parts by weight of EVA resin (vinyl acetate content: 32 parts by weight) and uniformly dispersed using two rolls at 60 C. The resulting EVA resin compound was placed in a press machine at 90 C. and allowed to stand for 3 minutes. The compound was held under a pressure of 5 kg/cm.sup.2, 10 kg/cm.sup.2, 20 kg/cm.sup.2, 30 kg/cm.sup.2, 40 kg/cm.sup.2, or 50 kg/cm.sup.2 for 2 minutes to form a sheet (9.5 cm long7.5 cm wide).

(4) Each sheet was crosslinked under the conditions of 150 C., 40 minutes, and a load of 21 g/cm.sup.2. The amount of voids formed in the sheet was visually determined. Discoloration of the EVA resin, which is particularly important for solar battery applications, was also visually evaluated. Table 1 shows the results of the test.

(5) Measurement of Gel Fraction

(6) Gel fraction can be used as a measure of crosslinking. A higher gel fraction indicates better crosslink characteristics. The gel fraction was determined by refluxing 0.5 g of the crosslinked EVA resin in xylene for 16 hours. The insoluble fraction was collected by filtration, dried, and then weighed. The gel fraction is the weight of the dried residue divided by the initial weight of the resin. Table 1 shows the results of the test.

EXAMPLE 2

(7) Example 1 was repeated, except that 1.2 parts by weight of 1,1-dimethylpropyl 1,1,3,3-tetramethylbutyl peroxide (abbreviation: POP) were used instead of the organic peroxide (BOP) used in Example 1 and that the occurrence of voids was not observed. Table 1 shows the results of the test.

COMPARATIVE EXAMPLE 1

(8) Example 1 was repeated, except that 0.8 parts by weight of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (abbreviation: DMDTB) was used instead of the organic peroxide (BOP) used in Example 1. Table 1 shows the results of the test.

COMPARATIVE EXAMPLE 2

(9) Example 1 was repeated, except that 1.4 parts by weight of di(1,1,3,3-tetramethylbutyl) peroxide (abbreviation: DOP) was used instead of the organic peroxide (BOP) used in Example 1 and that the crosslinking time was set at 30 minutes in the examination of the occurrence of voids. Table 1 shows the results of the test.

(10) TABLE-US-00001 TABLE 1 Comparative Comparative Example 1 Example 2 Example 1 Example 2 Components (parts by weight)* EVA resin 100 100 100 100 BOP 1.1 POP 1.2 DMDTB 0.8 DOP 1.4 Crosslink characteristics Crosslinking 160 160 160 160 temperature [ C.] T10 [seconds] 69 48 105 42 T90 [seconds] 945 421 2006 246 T90 T10 876 373 1901 204 [seconds] Crosslinking rate x Gel fraction [%] 93 86 94 77 Degree of x crosslinking Number of voids 41 43 127 [/10 cm2] Voids x Discoloration *The crosslinking agent was added in such an amount that the same number of peroxide bonds (OO) were provided. Crosslinking rate : T90 T10 is 400 seconds or less (considerably high rate). : T90 T10 is more than 400 seconds to 1,000 seconds (high rate). : T90 T10 is more than 1,000 seconds to 1,600 seconds (slightly low rate). x: T90 T10 is more than 1,600 seconds (low rate). Voids : The number [/10 cm.sup.2] of voids is 60 or less (considerably small number). : The number [/10 cm.sup.2] of voids is more than 60 to 80 (small number). : The number [/10 cm.sup.2] of voids is more than 80 to 100 (slightly large number). x: The number [/10 cm.sup.2] of voids is more than 100 (large number). Degree of crosslinking : The gel fraction is 90% or more (considerably high degree). : The gel fraction is 85% or more (high degree). : The gel fraction is 80% or more (slightly low degree). x: The gel fraction is 80% or less (low degree). Discoloration : Discoloration is not observed at all. : Discoloration is slightly observed. : Discoloration is clearly observed. x: Considerably significant discoloration is observed.

(11) The results show that when an organic peroxide having the structure according to the invention is used as a crosslinking agent, it takes only a very short period of time to crosslink EVA resin, and a crosslinked product having less voids and a good appearance can be obtained.